Yūichirō Hyakuya

Yūichirō Hyakuya (百夜 優一郎), birth name Yūichirō Amane (天音 優一郎), is a character in Seraph of the End. The main protagonist of the Seraph of the End: Vampire Reign manga, Yu along with Mikaela Hyakuya, possesses the "seraph" gene. During an escape from the vampire city they were raised as livestock in, Yu was the only one to escape from a trap by Ferid Bathory that resulted in his orphanage family being killed, where he also had to leave Mika behind.

Motivated to enact vengeance against vampires, four years later Yu becomes a member of the Japanese Imperial Demon Army, and joins Shinoa Hīragi's squad in the Moon Demon Company. Trained in part by Guren Ichinose, in combat Yu wields a Cursed Gear sword after making a contract with the demon, Asuramaru. Battling alongside fellow squad members Yoichi Saotome as well as Shihō Kimizuki and Mitsuba Sangū, after vampires launch major assaults on human cities, Yu would come into contact with Mika again.

Pursuing power to help save others, Yu developed a strong sense of looking out for family members. His swordsmanship would allow him to generate demonic powers to help survive and overcome foes. Yu also appears in prequel materials Seraph of the End: Guren Ichinose: Catastrophe at Sixteen and Seraph of the End: The Story of Vampire Michaela. Unknowingly a part of, and caught up in other's long term plots, Yu would have no memory that a child Yu existed in the ancient world who was involved with the first vampire, who became Shikama Dōji.

Human
Yūichirō has medium length black hair which can be considered quite neat and messy at the same time. He possesses sharp green eyes which fall under his thick eyebrows. His eyes are large compared to most other males. As a livestock, he wears the standard white uniform with short sleeves and a hood.

In chapter 2 of Vampire Reign when he is sent to school for his behavior, Yu is shown wearing his school uniform, with a standing collar that is black with teal trim. The collar and the top buttons of the jacket are unbuttoned, revealing is a white shirt underneath. He wielded a standard demon katana until he was promoted into the Moon Demon Company.

As a private of the Demon Army, Yu wears the standard JIDA uniform. This consists of a long-sleeved black coat with light green details including the which has a brown  passing diagonally over his right shoulder and two rows of gold buttons on the front. Along with that, he wears a pair of white gloves and a white web belt with a gold waist-plate. He also wears the uniform trousers and black boots which go over the trouser's leggings. Yu keeps his katana on the left side of his hip. Sometimes he can be seen wearing the JIDA hat, and often wears the JIDA cape as well.

Demon
Yu's eyes became scarlet, sprouts two black horns from his head, and his teeth become pointed while curse marks appear on the side of his face. At its most extreme his hand can briefly appear inhuman.

Seraph of the End
In his incomplete seraph form, Yu's left eye turns dark and bleeds while a massive wing-like structure sprouts from his upper left shoulder. As a full seraph, Yu has a single pair of irregular black wings as a result of Asuramaru trying to prevent the transformations, both of his sclerae are dark with gold irises, and the edges of his hair become white. The past Yu had six white feathered seraph wings. He was willing and able to slice off his own arm.

Human
Up until his teenage years, Yu was moody and antisocial due to the tragedies during his childhood. As a young child, he saw himself as a demon that no one would love, views largely shaped by his parents attempting to murder him. At first, Yu wanted nothing to do with Mika and the other Hyakuya orphans when invited to be a part of their family having had prior experiences with what family was to him. After spending time with others, and when placed in charge by Mika of watching over the kids alongside him and Akane, he assumed a role of the aloof, yet kindhearted older brother. With their loss, Yu reverted back to being a moody, distant rebel with a thirst for revenge against the vampires that killed his family for another four years and would resist making bonds with others.

Eventually, when Yu opens himself up and becomes a hot-blooded youth with a kind heart. This becomes apparent when his fearless but somewhat reckless actions are shown when he is attacked by a Horseman. He ignored orders, evidently on more than one occasion. When reports broke of an escaped vampire, Yu disregarded the warnings to avoid it, actively sought it out and engaged in combat using a regular sword. Not particularly studious, and tending to rush into things, Yu is often called an idiot by those who know him. Capable of being a very narrow-minded person, his only purpose was to annihilate all vampires because of what they did to his orphanage family, especially Mika. He will take on any type of challenge in order to accomplish his goal. Aware of the risks, he still acted on behalf of a possessed student to wrest a cursed axe away and was not concerned with the danger to himself.

"What was it you guys said? We humans are 'ugly'? Trash that would sell out anybody if our family's been taken hostage? Ha! Sounds like you're real familiar with that little ploy. You're right. Humans will do anything for their families. We'll happily lie, cheat, make deals with the devil, or even become demons ourselves. If you wanna call that 'ugly,' then fear human ugliness when it kills you, Bloodsuckers!"

- To the enemy vampires in Omotesando Station.

For all his anger, Yu does express worry about others. He was against Yoichi living for revenge due to his temperament, even though it was what he himself was doing. Even with is clear it was an illusion, Yu was concerned for the apparition of Mika who fell to the floor. Yu empathizes with others who put their family first, as when a civilian girl gave his squad false information because vampires are holding her family hostage, realizing why she deceived them, he tells the girl that she did the right thing to save her family. Yu bears a great deal of guilt feeling he left Mika behind to die, and would experience nightmares about Mika's death from that point on. He is overcome with tears when Mika appears heavily injured, and withdrew into himself when thoughts of not wanting to be alive anymore returned. While he focused on surviving until he could find someone to live for, Yu speaks how he never could really move on.

"I've lived enough. I considered suicide every day... But I stuck it out, doing what it took to live this long. You could at least praise me for that."

- Discussing a difficult topic with Mika.

After learning Mika is alive, Yu's personality changes in response and he grows more mature, as noted by Shinoa. He puts more effort into being patient and calm when making friends. His drive to kill all vampires worldview is immediately amended to saving Mika. Where Mika's vampirism was concerned, Yu was only worried it was his fault for leaving him behind. Regardless, even though his best friend is now a vampire, Yu still slaughters others without mercy or hesitation as he remains ready to deal with threats. After the events in Shinjuku, Yu becomes closer to his squad members. Ordered to join Kureto, his response is for him to free Yoichi and Kimizuki first who were held captive. Directly and indirectly, Yu seeks his squad's help in reclaiming Mika. Raising that Mika will be useful to Guren is used as leverage should it be needed, as he comes from the same orphanage as him. Additionally, friendship and bonds become more important to Yu, as shown in a mental battle against Asuramaru. To gain more power or else lose his body, he calls Asuramaru his friend and also turns to him for help is rescuing Mika.

Rather than revenge, Yu's primary goal extends to protecting his new family, and others he is working with in order to avoid a repeat of past events. In Nagoya, Yu surprises Narumi's squad having memorized all of their names and promising to protect them. Similarly to how vampires do not remember human faces and only think of them as "humans" or "livestock," Yu only sees vampires as "vampires" or "bloodsuckers" without bothering to remember their names or faces. He does not even recall the name of his vampire target in Nagoya. While he is calmer around others and more willing to help, Yu's drive to defeat vampires of increasingly stronger ranks does not diminish. His drive to protect family resulted in him partially becoming a demon when instigating a second rescue attempt, where the first one to save Guren from vampire nobles had already failed. When it comes to family, Yu feels anything is fine when they do it.

"If we tried our hardest to live... ...Then we just did what we had to do, and that's fine! Because we're family! Isn't that right, guys?! Tell me!"

- Justifying past actions no matter how bad since it is them.

He is happy and appears content in sharing with Mika how they are and have a family once more.

The Yu during the Dark Ages of Greece shared an exuberance. He expresses curiosity over minor or major things such as wanting to see the "Mikaela". He was happy to play with toys, and invited Ashera to take part too. Unusually absent-minded, Yu would give some thought when answering questions posed to him. When attempting to find something out, Yu asked questions that if true, the answerer would likely not want to confirm them. Asking Sika Madu bluntly whether he is a slave, Yu of the present would also have this trait when questioning Guren if he is being lied to.

In terms of what Yu is enthusiastic about, he was lively when watching Kimizuki hotwire a truck and was very keen to learn how to drive. Aside from this, Yu shows to have a somewhat specific sense of humor, as shown when he teases Shinoa for her height. After overcoming the shock of seeing them, the fear was also quickly gone as Yu took to marveling at the dinosaurs present in a memory.

"Wooow! Mika! Real, live dinosaurs!"

- Excitement at the dinosaurs around within a memory of the past.

Unfamiliar, indeed alien settings to Yu are no hindrance in his excitement. Although surprised to find in a memory that a seraph civilization existed during Earth's prehistory, after he had absorbed the scene his interest in exploring took over. Although he and Mika were seeking out where Mikaela went, Yu's means of achieving that was to go in every building to find him where he had an interest in exploring.

"Wanna wander around the place, looking?"

- Keen to explore a dramatically new environemnt with Mika.

When sparring, Yu also enjoys the combat. With the threat of an explosive spell tag stuck to his head, Yu embraced the chance to take someone down with him. He eagerly awaited Narumi's verdict on how well he and his squad did in a joint mission together. Against a vastly stronger foe, Yu eagerly requested one more round. Noting that there were two foes against him, Yu grinned adamantly that he definitely could kill them both.

"Two enemies at once? Can I manage them? Yeah. I can. I'll kill 'em all."

- Confident in himself when stood in between black demons Gekkouin and Kiseki-o.

Relishing the idea of a fight, Yu would show and speak more than once on his excitement. Not be daunted by odds against him, rather Yu would also dismiss them.

"Yikes! Only four more? Man, Guren sure is powerful!"

- Appearing determined, Yu mockingly considers how many more clashes he and Mika can endure.

Moving forward, Yu is against the idea of others' blaming themselves such as when Guren and Shinoa felt responsible for the catastrophe's effects.

"All that blame talk is meaningless! Being all "it's my fault," "no it's mine"... That's dumb! What the heck are you even doing?!"

- Yu shouts how Guren and Shinoa should not be blaming themselves.

Although not regarded as intelligent, and certainly struggling academically, Yu has demonstrated a cleverness in subterfuge. Deception seems to come from instinct and Yu is also astute enough to know when to deploy it. Apparently coming under Shikama Doji's influence, Yu indicated to Asuramaru that he still had his mind to then attack the First. Suffering a loss against demon Mika, while he was perusing his memories, Yu struck when he was engrossed in them to successfully strike him. Compromised or not, Yu benefits when he behaves as though he is defeated. Appearing as a monstrous demon out of his mind, it was enough to speak as he would do normally to convince Asuramaru he had returned to his senses.

"Sorry Ashera. I wore you out. By the way... ...I've been sane this whole time."

- After murderously ordering Asuramaru to give power yet wounded, Yu gives a convincing display that he remembers his squad are there for him, before revealing he was not out his mind before taking the chance to drain his power.

Away from danger, Yu enjoys activities that give him a sense of joy. Surmounting a tower by climbing it took dedication yet is it simply enjoying the sight that Yu was after. He was eager for Mika to come and enjoy the sight with him.

"Because if I stand up here on the very top... ...Doesn't that make me the tallest person ever?"

- A sense of jubilance at being at the top of a tower after getting out of a tense situation.

Casually discussing attacking settlements in order to gain supplies, Yu gives some thought to the wellbeing of citizens but is moved by what the larger picture entails. That he believes a resurrection he and Mika are planning can absolve wrongdoings factors into his willingness for others to perish for them to reach that.

"No. I'll do it. If they starve, I'll just revive them later."

- A willingness to take drastic measures, made easier in the belief that hard decisions can be remedied.

A commitment to defending family remains in the forefront of Yu's mind even when he is in a demonic state. At the same time, Yu tactically imparts such advice, while acting to protect such family. The strength he was employing at the time was a spell tag illusion where the copy acted elsewhere, to allow Mika time from where he was located.

"Dammit! See that, kid? You can't protect your family if you aren't strong enough. You'd better get stronger, or else!"

- Where he told a boy to get stronger to defend his sisters, Yu sees his current strength cannot help defend his own family.

An initial desire for revenge left Yu not afraid of, but willing to battle extremely powerful foes such as vampires. With that goal, Yu was similarly fearless about demons, who he regarded as a means of acquiring power. Both known and unknown forces did little to deter Yu from acting towards his goals. For potential repercussions from stronger entities, the fear of retribution was ineffective on Yu.

"We've already been hit with all kinds of divine punishment. The whole world's wrecked too. What's it matter?"

- Unafraid of the repercussions of sacrilege given that they have already occurred, and happy enjoying himself doing nice things.

Fear also did not prevent Yu from enjoying himself. After scaling a mountain, Yu was content seeing the view before him from atop a shrine. Asked if that was not sacrilegious, Yu remained brave and continued to enjoy the views he was there for.

"There, see? If we've gotta live, might as well have fun."

- Responding to Mika who said it has been a while since they had fun.

Seraph of the End
During his first transformation, Yu goes berserk and attempts to kill any human nearby. He displays no interest in vampires and recognizes no one, but he does tell a vampire to not interfere. He also mutters that "sinners must die", and he must kill all sinners as he advances towards Shinoa with a sword. At being embraced by her, partial seraph Yu reacts with anger.

Upon his second transformation, influenced by the King of Salt seraph, Yu curses the tainted humans and plans to turn them into "pillars of salt." He defends himself from Abaddon, a monster summoned by another seraph. Then acts to strike down the monster before addressing the humans.

During the Dark Ages of Greece, child Yu as a seraph displays the same murderous behavior as the present day Yu though to a more unhinged extent. Neither slowly advancing, or take his time to speak of the sinners, the past Yu sprung towards Ashera in a bid to murder him. Undeterred by words to try and calm himself, neither being restrained, Yu was overcome by a need to kill to the point where he freed himself, and broke through doors to stab into Ashera. He then took to the skies carrying Ashera upwards an rambling madly about murdering sinners.

Namanari
In Yu's demon form, he alternates between earnestly spoken deception and furiously voiced threats to get what he wants. Attempts to persuade Mika into freeing him include enticing him with his blood, and promises for them to run away together if only he would set him free. With him not being freed from his restraints, Yu wants to kill and destroy anyone nearby. He screams and rages that he will devour their souls. When Yu regains his human mind, he does not recall any of his actions as a demon. Despite this, he appears utterly nonplussed about losing his humanity.

Until he receives medication for keeping the demon side restrained, Yu is observed by his squad to have been acting colder about everything. According to Asuramaru, he is able to think much more rationally without all of those emotions in the way. While Yu could consider allying with vampires they could not defeat, and whose information they needed, Yu was instantly swallowed in a murderous rage at seeing a deeply unsettling sight. After receiving the medication, Yu's personality returns to how it was before he became a namanari.

While drawing on demon power with demonic horns and fangs himself, Yu can still retain his sanity despite his murderous outer appearance. Where Asuramaru was none the wiser to this, Yu used the opportunity to vanguish him as a threat. Fighting Guren, he could prevent his sword from a boy used as a human shield. In his demonic state, Yu would express the intent to kill Guren no matter the cost.

"Ugh, so hungry... No strength left... Could've had a chance if I'd killed him through the kid..."

- Reflecting he had a chance to kill Guren.

Ambitions
Before the calamity, Yu strived for nothing as a child where he was reacting to his parents having tried to murder him. Brought underground to live as livestock where his blood would be taken by vampires would take its toll. From this treatment, and the way he was seen, it would result in Yu forming a goal of fighting them. It was not only for his own sake, but for others as well.

"GAH! I CAN'T TAKE THIS ANYMORE!! It hurts! It makes me dizzy! And I can't stand those blood-suckers looking at me like I'm livestock! What do they think we are? Pigs? Cows? Chickens?! That's why we've gotta start a revolution! You and me, Mika. We'll both get stronger, and then we'll go kill the vampire queen and turn this place into a human kingdom!"

- Twelve-year-old Yu to Mika in the streets of Sanguinem.

The loss of his family would instil an unwavering murderous intent to kill every vampire. The commitment to this would drive Yu to become proficient in using swords. Dedicated to training, and not daunted by the intensity of the sparring practices, Yu focused on little else but the goal of acquiring vengeance. Undeterred, it was a burning ambition that did not subside with time.

"Yes... That's what I want. If it means I get to kill those blood-suckers, I'll do anything!"

- A vow to enact vengeance, that Yu would abide by for the next four years

To this end, Yu worked towards acquiring the means to fulfil his singular task. Entrance to the Moon Demon Company organization was one route, so Yu aspired to join that. Aware of what Cursed Gear could do, Yu simultaneously concentrated on gaining a weapon that could challenge the supremacy of vampires. Naive as to what was entailed, Yu saw Shinoa's cursed gear scythe, and at once told her to hand it over. Fighting with his regular weapon was done in an attempt to get it. Similarly, in attempting to seize a cursed gear weapon, Yu would come into his first contact with a demon and seek to make a contract with such a being. For his vengeance, Yu successfully achieved his aim of joining the Moon Demon Company, and made a pact with the demon Asuramaru.

With the means to actually fight vampires, with decreased probabilities of being killed, Yu set about killing them, often on sight. Where he was a part of Shinoa squad, Yu's goals had expanded to also fighting to defend them. His past experiences shaping his future desires now that he was close to people once more, Yu would apply this protect others approach to strangers as well.

"I don't ever wanna say I survived because I left someone else to die ever again!!"

- To his squad when he enters an obvious trap to save a human girl.

A continued need for power was there since vampires, particularly nobles, ranged in strength. Nevertheless, Yu sustains his fight against them. Striking one who was about to kill Guren, he found it was Mika. An unyielding hatred for vampires, that persisted and strengthened itself over years was gone where Mika was concerned, and Yu instead vowed to save him. That Mika was alive was cause for Yu to abandon a need for vengeance, and instead gain the means to rescue Mika. For this, for the first time in a long time, Yu took to reading to learn what he could about changing vampires back to human. Additionally, Yu sought the assistance of his squad, and Guren to help reclaim Mika from the vampires.

"...I was looking west. That's where Mika's gotta be, right? In the vampire city. I heard the place they held us was somewhere underneath Kyoto... Someday, I swear... I'll go get him back."

- Speaking to Shinoa when asked what he was looking at, Yu would seek her's, and others' help to reclaim Mika.

Found by Mika a second time, Yu focused on having him drink his blood to recover from injuries. It would transpire that before, Yu did not want to live after he lost his family, believing that included Mika, yet held the concept that he should continue to do so in case of a future event that someone would need him. At the time, both vengeance, and holding out for this scenario sustained Yu. This fidelity to saving Mika would be the first prolonged desire Yu had apart from enacting vengeance against vampires.

"Years ago, Guren told me something. Oh! You wouldn't know him. He's the guy who took me in after I escaped on my own. I'd just lost all of my family. I was hurting so bad inside, and I wanted it all to end. I wanted to throw it all away and give up on life. But he told me that, even if my only reason was for revenge, I should live. So then... Maybe someday, somewhere... I'd find somebody who would need me again. So I gave the whole living thing a chance. Even though I figured no one would ever need me, I kept surviving. Struggling, flailing around like a total idiot... But apparently I kept at it long enough that I found you again. Mika... let's live on together, okay?"

- Detailing his reasons for living to Mika, and seeing him as the one he was awaiting.

Open to the idea of running away with him, Yu intended to check that his squad were okay before doing so where their possible help was also considered. With recovering others, Yu was very keen on the idea of resurrecting humanity when hearing of that idea from Guren. Never minding the costs, Yu did not consider the potential of risks involved either. Merging this prospect of resurrection, to have it overlap with returning Mika to being human would be an elated aim of Yu's.

"I know. And I want to save you. I promise you, Mika. Even if I have to sell out the whole world to do it, I'll make sure you're turned back into a human. Akane. The other kids. Narumi's family. Kimizuki's sister. I'll bring everyone back."

- Sharing his resolve to Mika and their squad.

For the plot they were increasingly aware of being involved in, Yu also desired to learn what the details were. Needing information Ferid had, he would also work on honing the seraph capabilities within him to battle Ky Luc in order to free him and Krul Tepes. Taking chances when the opportunity arose, Yu also sought information on how to turn Mika back from the Japanese Imperial Demon Army. Snared by events outside his control, Yu was willing to go along with what Guren, who had attacked him, wanted as he was told there was a way to save Mika who was now a demon. Steadfast, Yu would fight his own demon, Asuramaru, and the possession attempt he brought on in order to help Mika.

"My... ...Greatest desire... ...Is to save Mika."

- Being possessed and asked what his greatest desire and fear is, Yu focuses on one and states it.

After they came under attack from the First, he sought to have Mika form a contract with him to prevent his capture. During this time where Demon Mika was accessing the deepest point of his heart, he appeared as Mika would as a human teenager in an indication of what Yu's innermost desires are. A notion imagined to generate power, Yu spoke how he wanted others to love him given that he could not love himself. Succeeding in bringing Mika back where he was now a demon with his memories returned, Yu was enthusiastic about them both seeing what they could try given Mika's new capabilities. Upon gathering that Guren had a hand in his family's murder, a strong resonating yearning was to surpass him. Both a means of providing power for Mika, it may have also reflected Yu's genuine sentiment.

"Yeah, let's do this! We can be even more powerful yet!"

- Determined for him and Mika to win against Guren and Mahiru.

The notion of family shaped what Yu wanted, where he forgave Guren for his involvement. Where there would be a scenario on resurrecting family members who had been gathered, there was also an ultimatum. From the choice of resurrecting his orphanage family, including the rest of humanity who had fallen, or to restore Mika alone, Yu fought for the latter option. For all his happiness at the idea of bringing humanity back, to the resolute determination to do so, with an intransigent disregard for the consequences, Yu placed the chance to restore Mika's humanity before that.

"I only want one thing. And that's to protect my family... ...From this craptacular world. Help me with it, okay? C'mon."

- Laying out his path to Mika, where family seems to apply solely to him.

Knowing that his squad were there for him, Yu maintains his goal to protect family, yet in leaving his squad it seems the case the family related words concerns Mika foremost. Confirming this in person, it is when Yu is alone with Mika does he take the step of forming plans by themselves where before they have been taking directions from other.

"We haven't decided anything yet. We haven't thought for ourselves one bit. We've only done what Guren or the First told us to do."

- An initiative of forming a course of action with Mika.

To this end, Yu is firm in remaining with Mika. Even when outright commanded to return to his squad, and make a family with Shinoa to seek happiness, Yu feels he cannot start a new life if Mika is not in it. Mika is also seen as the one who can help with a joint plan of their making.

"I've zero intent to live all on my own... ...So you'd better use those smarts of yours. Come up with a plan that lets us win."

- Firm in being with Mika, and seeing him as someone to help enact their plan.

Meaning to explore ancient memories, where a food supply is needed in order for Yu to generate desires that would help Mika see into the past, Yu settles on attacking human settlements for supplies. Undeterred at the possibility of others dying, even if ressurection is not an option, Yu is unwavering in meaning to defend his family.

"I've made my choice. For my family's sake I'll become a demon, or even the devil himself."

- Sharing his resolve.

Enjoying a sense of freedom, Yu took to climbing high reaching structures. In reaching the top of Toktoy Tower to enjoy the view, Yu also gave some thought to all the people he was going to resurrect. An identical desire was applied to Mt. Fuji.

"But I want to climb it. Just look at it! Wouldn't anybody want to?"

- Having sought out Mt. Fuji for the joy of climbing it.

Willing to climb such places even with the risk of being spotted by foes, Yu was motivated to enjoy himself nevertheless. Sharing why with Mika, he mentioned with battles ahead they could die so they should try and create some nice memories. Although at the time Yu considered battles, he assured Mika of nicer times yet.

"Yep! We're gonna save the world while having fun! By the way, I'm staying up here until I get to see my first dawn too."

- His intention for the long, and short term where he will enjoy it as well.

Story
For the total coverage story detailing Yu's history as well, see.

Yūichirō Hyakuya was brought to the Hyakuya Orphanage after his parents attempted to murder him and there meets Mikaela Hyakuya as well as other orphans that lived there. An apocalyptic event occurred where the adult population perished and vampires emerged to gather the surviving children. Yu was taken to an underground vampire city where he lived for the next four years. Discontent with living there, Yu would realize that Mika was giving his blood to vampire noble Ferid Bathory. Angered further, Yu would tell Akane Hyakuya that he had no family. His desire to leave would result in Mika bringing back a map with an exit to the city. The plan for Yu and the orphans he lived with to escape would be successful, until the end where at the exit Ferid revealed it as a cruel ruse and set about murdering the orphans. With Mika sacrificing himself, Yu reluctantly left him behind to escape having recognized they were his family. Outside he would be found by Guren Ichinose and swear vengeance against vampires.

Four years later, Yu patrols outside as a scout but with an unwillingness to work with others he is suspended until he can make a friend in the education system. He is monitored by Shinoa Hīragi, where at her urging he helps Yoichi Saotome against bullies and engages an escaped vampire. After coming into contact with a cursed gear weapon and overcoming a demon that had taken the form of Mika and their orphanage family, Yu was told he would be able to join the vampire extermination unit. Coming into conflict with Shihō Kimizuki, he would continue training until selected to choose a black demon ranked cursed gear. The katana sword he touched would bring him into contact with Asuramaru, whose strength Yu needs to enact revenge and protect others. Haunted by dreams of Mika asking why he left them behind, Yu would head out of the city to track vampires and see Mitsuba Sangū become the fifth member of the squad. After rescuing a child from a Four Horsemen of John, and resisting the incoming vampire trap, Yu would locate their stronghold and free the humans kept there.

Proceeding to Shinjuku to find it under attack, outside the walls Yu confronts a vampire noble, Crowley Eusford who is joined by Chess Belle and Horn Skuld. Within the city Yu would stab a vampire who had Guren cornered to then realize that it was Mika who had survived. Yu would transform into a monstrous one winged being and then pass out. With the battle won in humanity's favor, Yu was relieved his squad and now Mika were alive. Determined to rescue Mika from the vampires, Yu would be summoned by Kureto Hīragi of the Japanese Imperial Demon Army. With rapid advancements in battling capabilities after training, Yu would hear from his squad that he had likely been experimented on and that it was possible Guren was using them. He decided to stick with Guren as a part of their family.

Before a campaign to eliminate vampire nobles in Nagoya, Yu would find his own prowess and team work lacking compared to others. Along with Makoto Narumi's squad and Shinya Hīragi they would collectively defeat fifteenth progenitor Lucal Wesker. Other missions across the city were not so successful and Yu would take part in rescuing hostages captured by Crowley and the vampires under his command. After a rescue attempt involving Guren, Yu would become a partial demon before being having to be rendered unconscious by his team with vampire reinforcements arriving. Carried to the airport, Yu would be retrieved by Mika. Upon awakening he was further warned that the humans were using him yet Yu would not entertain the matter until Mika drank his blood to survive his injuries. With Mika he arrived at the airport to find the survivors of the mission being executed by Kureto and defend his squad from the army and an incoming fleet of vampire forces. The experiment culminated into a seraph gaining consciousness in the world which summoned the demon Abaddon. In an attempt to save everyone, Yu resorted to the inhuman power within himself and as the King of Salt was able to vanquish this threat.

Three months later after escaping the army and vampires with Mika and Narumi as well, living near the ocean Yu would be afflicted by prolonged durations of becoming a demon. With finding Guren and Kimizuki's sister Mirai Kimizuki a goal, Yu agreed with Mika's plan to rescue the vampire queen Krul Tepes who was betrayed by her own kind to assist with this. Yu and the squad in turn would be located by Ferid and Crowley where unable to defeat them, had little choice but to adhere to their plans. Seeing Ferid reveal Akane's head would result in Yu succumbing to his demon of which the problem was dealt with using a syringe Ferid had. Travelling to Osaka Bay, Yu comes across a huge number of vampire nobles who have arrived in Japan from foreign countries. With Ferid and Krul sentenced, Yu and the squad head to Ferid's mansion with Crowley since using the Seraph of the End power is all that can be resorted to in order to save who they need. After finding his deceased orphanage family members kept in containers, there Yu would also be joined by Guren.

With one noble, Ky Luc, guarding the bound Ferid and Krul, Yu would join in the attack to free them where he could deploy his abilities as a seraph, which resulted in Ky Luc retreating with Krul. A childhood memory surfaces involving Mahiru Hīragi and Yu awakens at Guren's residence. There, another seraph is kept who views the King of Salt as a traitor for not destroying the world. Coming under attack, Yu manages to subdue the seraph. Joining Guren with the squad at Shibuya, Yu offers his memories if they can help Shinoa against her demon Shikama Dōji who is decidedly a threat to them. After he is led away, Yu is kidnapped by a member of the Hyakuya Sect that has launched a stealth attack against the city. Finding himself being strapped and wheeled away on a stretcher, Yu is saved by Mika before seeing Guren with horns before him. They come under attack by Guren and his demons. Although Yu is half possessed and manifests some formidable powers, he is unable to prevent himself being neutralized as a threat, neither can he stop Mika from being hurt. Crying as Mika rises into the air, Yu witnesses a explosion envelop Mika. He is unwilling to leave until persuaded to by Krul who arrives and states that Yu can save Mika later if he leaves now.

Some distance away from Shibuya, Yu has escaped with his squad and Krul. He sees that Shikama Doji is in control of Shinoa's body as Guren joins them shouting for Yu to turn Mika into cursed gear. Hearing from Shinoa that Asuramaru is allied with the First, when Yu gets no answer he concludes this is the case. Finding Guren creating a large spell circle, Yu is to use it to reach Mika who he has heard call for him. Touching Mika's sword results in him breaking out of Asuramaru's chest in a demon realm. Determining that he is the enemy, Yu answers his desire is to save Mika, and as a demon with three seraphs behind him he seals Asuramaru before using his newfound power to vanquish Gekkouin and Kiseki-o. After reaching Demon Mika and waking in Nagoya with a red sky, Yu asks why Mika does not remember him when this is the location where they met again. Rather than Demon Mika possessing him, Yu means to save him and fights to do so. Able to ambush Demon Mika, he sees they share the same deepest trauma of the loss of their family. When Yu is decapitated he contacts Mika who has been found in a memory of the ancient past by Sika Madu. Saying for them to protect family together, Yu asks Demon Mika to become his demon. After fending off Shikama Doji, at returning to the physical world Yu turns to the matter of Guren's plan.

Travelling to Akihabara, and speaking privately with Mika, he has questions for Guren. Asking him upfront whether Guren had a hand in killing his orphanage family having had suspicions, Yu takes to battling him on the spot. While enraged and wishing to surpass Guren's capabilities, Yu decides to forgive Guren. Reaching an underground facility where the deceased are kept, Yu chooses to fight for Mika when there is a choice to restore him, or resurrect all of humanity. In a demonic state, Yu deceives Asuramaru and consumes his power to escape the vampires around him that he could not previously outmanoeuvre. Having a different resurrection method to discover, Yu travels with Mika alone to Mt. Fuji to search memories of their past.

Natural abilities
Yūichirō is an excellent swordsman, being trained by Guren himself. He is capable of easily taking down Horsemen with both an enchanted gear and Cursed Gear blades, and has proven to effectively fight and slay common vampires. Closing a distance and attacking fast and hard before they or others can react has proven very effective. Yet another testament of his skills is that with just a regular blade, he was able to go up against Shinoa using Shikama Dōji, which earned him her praise. Along with that, later he can hold his own in a sword-fight with Guren, although not enough to actually match him equally as Guren was holding back due to it only being a training exercise. Yu was easily defeated by Kureto, a swordsman who was, although only by a slight margin, inferior to Guren in terms of skill. Nevertheless, he earned praise from Aoi Sangū for his sword skills, after pressuring her when she blocked his attempt on Kureto's life.

Upon synchronizing with his demon his own strength has greatly improved, although Yu has yet to control it with his demon as his own mind went fuzzy and is unstable, but he has demonstrated new techniques.

"We'll win in one hit! And we won't clash blades. It'll take just a second... No, an instant... A flash."

- Changing tactics.

In response to factors such as poison, Yu is willing to change tactics to help win. Remaining determined, Yu means to take the most efficient path to victory. Having the capabilities to climb a tower, Yu registers that he is no longer human in that he can survive the 1,000 feet fall.

"Ooh! I didn't die! Whoa! I fell all the way from up there... But I'm still totally fine!"

- Joyfully noting that he survived falling off a tower.

In viewing memories from prehistorical times Yu finds he is able to read an unfamiliar written language. It seemingly requires concentration and a sustained effort in order for the text to become legible.

"Aw, do I gotta? It's really hard and it's making me use my brain so much I'm already getting a headache. And boy does it use a ton of calories!"

- Although challenging, Yu finds he can read text he is not familiar with.

Experimentation
By using the golden trumpet within his consciousness, Yu undergoes a drastic transformation. As a completely awakened Seraph, he is the Second Trumpet referred to as the "King of Salt" (塩の王). His main power consists of turning every living being into pillars of salt. Additionally, he can also create a spear made of salt that can block Abaddon's destruction ray. He later has full control of his Seraph state.
 * Cursed Gear Stimulants: Like other soldiers in the Japanese Imperial Demon Army, Yu takes a supplement to increase his synchronicity with his demon to 150% for 15 minutes. After that time is up, this synchronicity drops to zero. There are numerous unpleasant side-effects including one's internal organs exploding with a larger number of stimulants. Yu's seem to have been given to him to help activate the seraph within in dire times such as a city invasion.
 * Seraph of the End: Yu is also a "Seraph of the End" hybrid with 1/10th of him being not human due to him and his old family being experimented on by the Hyakuya Sect. His seraph powers give him greatly enhanced speed and strength. After he overdoses on the curse supplement to save Guren from Crowley, he loses his mind and undergoes a painful transformation. Although extremely powerful, Yu goes berserk and attempts to kill any human nearby. Guren causes this transformation to end by ordering Shinoa to hug Yu. The transformation ceases, leaving Yu in a coma for a week with his non-human part increases. It takes five days for the drug to get out of Yu's system, and even after that, he feels tired. Yu has no recollection of the events that occur while he is transformed. Later he would be able to control his own Seraph on his own.
 * Immense Physical Strength: As seen during his fight against Ky Luc, while in his Seraph form, Yu possess tremendous physical strength, as seen where he was able to physically damage and even break the limbs of a fifth Progenitor Vampire, greatly surprising Ky and making him actually doubt if he could overpower and defeat him.
 * Prehensile Tongue: While in this form, Yu's tongue becomes black, extendable and incredibly sharp, with him having been shown to use it to injure and cut into Ky Luc's neck.
 * Regeneration: While in this form, Yu's regenerative powers are also boosted to the level of Vampire Nobility, as seen where after Ky Luc's pierced his chest, crushed his heart and left a fist sized hole in his chest, Yu was able to not only survive the wound, but completely restore his lost organs at a rapid pace.

Enchanted Gear
Mass-produced cursed gear, these demon weapons are much weaker, usually of the yaksha or myo-o series, as they increase a regular human's strength by three to four times. Yu wielded one in the form of a katana, and was able to use this gear to easily slay a horsemen and hold his own against the escaped vampire.

Cursed Gear
These weapons are capable of increasing a person's strength by seven times.
 * Asuramaru (阿修羅丸): Asuramaru is a possession-type demon weapon of the Black Demon Series, where he takes the form of black katana with a green edge. Whilst in use, the blade develops a thick green aura. Like Mahiru Hīragi, he can call upon Asuramaru to fully use its power to attack opponents.
 * Asura-Kannon: A special technique using Asuramaru that materializes dozens of floating swords.
 * Asura-Kannon: Thousand Blades: An advanced type of Asura-Kannon, where a warped space is formed and expand around Asuramaru, which thousands of swords materialize and surface from the warped space with a homing effect at the target.
 * Asura-Kannon: Ten-Thousand Blades: Knot of Fate: When used, the original warped space range widens, able to cover an entire building, and thousands of swords emerge from the warped space which speed towards the enemy. It has enough destructive force to shatter the surrounding area in the warped space.
 * Mikaela Hyakuya (百夜 ミカエラ): Mikaela Hyakuya is a possession type demon, taking the form of a black sword that is ornate at its hilt and the end of its sheath.
 * First Edge: Pandemonium.: Generates a stream of shadow like energy from the blade that spills to the ground.
 * Second Edge: Black Gash.: Darkens the area around a target, where an affected area, such as the area around a head brightens. It warps the space around a target's head, shining brighter until the head more or less disappears.
 * Third Edge: Phantom Journey.: Resulted in demon Mika appearing outside the weapon to help withstand the First.
 * Flicker... Mikaela.: After electricity crackles around the hilt, streams of lightning surges out, where largely it is a singular jagged beam that fires across a great range.

Spell Tags
Having engaged with them in combat with Guren, spell tag creations of individuals is something Yu picks up on. Whether having an innate ability to generate them or familiar with them through other means, a ring of light suspended around Yu who levitates mid-air with streams of electricity coming from his fingers appears to help form such illusions. Two spell tag creations of himself, and even more notably a second individual of Mika can be created to act in a location far away from where he is.

"Mikaaa! The ruse is up! I can't keep this up any longer!"

- Calling to Mika where apparently his spell tag creations of them can no longer be sustained.

Book 5
"Who are you? Are you here to kill me, too?"

- Yu to Mahiru upon meeting her the first time.

"But I don’t! My mom and my dad said so! They said that I was a demon and needed to die! That as long as I live, bad things will keep happening… so instead I need to die! My dad was always so nice to me, but then he pulled that knife out… And then my mom said I shouldn’t be alive anymore. She said I’d be happier dead. Because I’m a demon, a monster, and I’ll ruin the world… It’s… It’s my fault that they’re dead… Mommy! Daddy!! Ahhh!"

- Yu to Mahiru.

"B-But… How is someone like me living worth anything?"

- Yu to Mahiru.

Seraph of the End: Vampire Reign
"Who needs some stupid 'family'? I got sent here because my father tried to kill me. My mom kept ranting and raving about how I was demon spawn. She eventually went nuts and killed herself. So do you get it now? 'Family' doesn't mean anything to me!"

- Eight-year-old Yu to Mika the day Yū arrives to the orphanage.

"Quit trying to shoulder all our burdens by yourself!"

- Twelve-year-old Yu to Mika in Sanguinem.

"You're... my family. I... finally found a real family! I can't leave you!"

- Twelve-year-old Yu to a fatally injured Mika.

"You aren't Mika. Mika would never tell me to get revenge. He was smart, but he was always stupidly nice to all of us. SO WHO ARE YOU?! HOW DARE YOU PRETEND TO BE MY FAMILY!! I'LL KILL YOU!!"

- Describing Mika to a Myo-o demon and angered it would take his form.

"ARE YOU STUPID?! WHO CARES ABOUT POINTS?! THIS IS YOUR FAMILY WE'RE TALKING ABOUT! If they die, then... they're gone for good. You won't ever see them again."

- Yu to Shihō Kimizuki when told that Mirai Kimizuki is in critical condition.

"Once I have the power, I'll get revenge for the death of my family. Those bloodsuckers killed Mika, so I'm gonna kill them all!! That's my one and only reason for living."

- Yu before first challenging Asuramaru.

"DAMN IT!! SHUT UP, SHUT UP, SHUT UP!! Just give me your power already!! I... need it!! I need enough power to protect everyone!!"

- Yu confessing the truth of his feelings while confronting Asuramaru.

"...'Those humans' are using me? What's that mean, Mika? You... You make it sound... like you aren't human anymore. It's my fault, isn't it? My fault for running away..."

- Yu to Mika when they reunite.

"S-s-ssi-sinners... S-sinners must... d-die. M-must kill... all s-si-sinners."

- Berserk Yu preparing to kill Shinoa.

"Do not interfere, Vampire. All humans must be killed."

- Berserk Yu speaks nonviolently to Mika when he protects Shinoa.

"It wasn't a dream. And that means Mika really is alive. Ha ha... He's alive... Ha ha ha... I'm not dreaming... He really didn't die... Thank God... I'm so glad!"

- Yu to himself, after waking from a coma.

"Just you wait, Mika. I swear... I'll save you."

- A vow to himself.

"Whatever your reasons, if you hadn't rescued me that day, I would've died. I do still feel like I owe you for that. So what I wanna know is... Am I actually useful to you? That's all. I don't care what you and that Hīragi guy are fighting about. I don't care what you're after. All I want to know is, do you need me? If you do... Would you want Mika, too? He was in the same orphanage as me. If you do... If you think you could need Mika like you need me... Then, I'll be your guinea pig or anything you want. I'll do everything I can to help you with whatever you need. Just tell me how I can get Mika back!""

- An attempt to involve Guren in Mika's rescue.

"Those words don't hurt me anymore. Besides, I don't care if I am demon spawn. If Mika needs me.. If Guren, Shinoa, Mitsuba, Yoichi, and Kimizuki want me as their friend, then I don't care if I'm a demon."

- To Asuramaru and unconcerned about being a demon.

"I don't care if you're a demon. I need you, Asuramaru. Join me, please. We can be friends. Help me rescue Mika... He's family."

- Requesting Asuramaru's help in a different manner.

"I... Um... This is kinda embarrassing, but... Well... Guren said we're family now, right? So you, Yoichi, Kimizuki, and Mitsuba... We're all a new family. And I've started to feel that's true. You guys are important to me. But... I can't forget Mika. He's also an important family member. See... My parents abandoned me. So maybe I don't really know how families work. But can I ask you guys this? Just because someone in your family betrays you, does that mean you have to betray them back? I don't... think it works that way. So even if Guren betrays us... and Mika, well... even if he is a vampire now... he's still family. We're all still family, and we always will be."

- To Shinoa squad when deciding who to ally with.

"So, if it's for the mission... should we have to ignore our feelings and abandon family? But, Shinoa... If we do that... If we just give up on family in this empty, ruined world... Then what? What else is there... to cling to for survival out here? Guren... He... He's our family!! Crap. I can't do it. Sorry, Guys. Looks like I'm just not cut out for that teamwork stuff. You all go on ahead. I'm gonna stay here. If I can't save my family... I'd rather die anyway."

- Yu to the Shinoa and Guren Squads before overdosing on curse stimulants.

"Why'd you save me? I... I don't have a family anymore... I haven't since I was born, really. My parents called me demon spawn and tried to kill me... I made a new family, and then they all got killed too. After all that... What's the point in living? Is it really worth it... for me to even bother?"

- Twelve-year-old Yu to Guren in a flashback.

"NO!! DON'T YOU DARE GIVE UP!! YOU... YOU WERE THE ONE WHO TOLD ME I HAD TO LIVE DESPITE ALL THE PAIN!! SO BE A DAMN ADULT AND BE A GOOD EXAMPLE FOR ME!!"

- Yu to Guren before becoming possessed.

"Every time I close my eyes, I see the same scene. I'm back in the underground world ruled by vampires... watching my family die. I'll make them pay. I swear, someday I'm gonna make them pay. I'm gonna find those bloodsuckers... and kill them all!!"

- Thirteen-year-old Yu's thoughts.

"Gimme all your weapons!!"

- Thirteen-year-old Yu to city guards.

"No more... I said I don't have any of those anymore!! You know what happened! My parents abandoned me!! I finally found a second family at my orphanage, but then they all got killed by vampires!! Anybody that I even start to like dies!! How am I supposed to find anything to protect anymore?!""

- Thirteen-year-old Yu to Guren.

"Mika, whoa! It's me! Yu! Don't you recognize me?!"

- His reaction to Mika attacking in a state of blood lust

"Who cares about that right now?! You look like you're about to die, Mika! What can I do?! How can I help you?! It's blood, right? Will you heal if you drink some? Then, go on and drink mine! Here! Take it!"

- Offering blood to help Mika.

"If you up and die on me again... I... I'll cry, you moron!! So live, dammit! Even if you have to turn into a vampire! Even if you have to crawl through the mud on your hands and knees--live!!"

- Trying other means to persuade Mika to drink.

"LA LA LA LA LA!! I can't hear you!! Who's gonna pay attention to some dumb idiot who just gives up on living like that! If you want me to listen, drink my blood first. If you want to save me, then let me save you first. You aren't going to die, Mika. I won't let you."

- Yu to Mika.

"Hah! Don't be stupid. Just drinking a little blood isn't enough to make you a monster. No matter what you are, we're still family."

- Dismissing Mika's concerns about feeling a monster, where it would not matter even if it were the case.

"Welcome home, Mika."

- Yu to Mika after Mika bites him.

"I died long ago on the day my parents abandoned me. But then you helped me live. And after you, it was Guren. Somehow, thanks to you two, I'm still here. And in that case, if either of you want to use me... fine. Go ahead. I don't mind."

- Telling Mika how he felt after his parents left him.

"I said I wanted to rescue my family. So I'm going to rescue them. And you're obligated to help me."

- With Mika safe Yu intends to rescue his squad with Mika's help

"What? Run away to somewhere both the vampires and the Demon Army won't ever find us? Man, that brings back memories. Back then we were both desperate to run away from our world of vampires and slavery. So we escaped... Hoping and praying to find some paradise where we could live in peace... But all we found was this. We can't run, Mika. There's nowhere to run to. No matter where we go, all we'll find is more humans and vampires."

- Considering escaping with Mika with the realities at hand.

"Yeah, yeah. Say that all you want... I'm still a human myself, y'know."

- Yu to Mika after Mika keeps harping on how terrifying humans are.

"Well, without her saving you, we never would've seen each other again! It's only right to be thankful to her for that."

- A favourable view of Krul Tepes spoken to Mika.

"Shinoa, I have no idea what's going on here, but we're all gonna help each other get out of this alive. Shinoa, what are our orders?"

- Reaching Nagoya airport where soldiers are being attacked and turning to Shinoa as the squad leader.

"I swear!! I'm gonna get us all outta here!! And I won't let any of you die!!"

- Meaning to help his squad escape the Japanese Imperial Demon Army and vampires.

"This... This can't be happening. Tell me this isn't happening, Guren. You'd never betray your friends and family, right?! There has to be a good reason for this!! Say something! Anything!! Guren!! Weren't... Weren't we... Weren't we your family?!"

- Trying to understand Guren for attacking.

"Mika... Guren... H-He was crying... We have to save him..."

- Telling Mika he feels Guren is still in there

"As long as I can protect my family, I don't really care."

- Yu opts for the trumpet over Asuramaru's power and lose his humanity altogether.

"The end has come. Tainted humans who dare reach for the forbidden... May you all become... PILLARS OF SALT."

- Yu awakened as a seraph.

"The end has come. On this day, the cycle of human greed and desire... shall crumble... as naught but pillars of salt in the wind."

- Yu as a seraph.

"SILENCE!! HUMANITY SEES ITS END THIS DAY!!"

- Yu as a seraph to Asuramaru.

"STOP!! Cease this, humans!! Do not reach further yet into the forbidden!!"

- Yu as a seraph before becoming a demon.

"LET ME GO!! UNCHAIN ME RIGHT NOW!! I'LL KILL YOU!! I'LL KILL ALL OF YOU!!"

- To Mika after becoming a full demon.

"Please... Mika... Take these chains off of me, wouldja? C'mon. We'll run away together. Just the two of us... You and me... We're the only family we've got. Right? ...Oh, I know! You've got to be getting thirsty, right? You can drink my blood. Then we'll run away together. We'll run away from all the humans... and all the vampires, and go live in peace—just the two of us. Isn't that what you want? ...C'mon, Mika. They hurt. Really. Take these chains and wards off. Please, Mika. DIDN'T YOU HEAR ME?! I SAID TAKE THESE DAMN THINGS OFF OF ME!! TAKE THEM OFF!! GET THEM OFF OF ME NOW!! I'LL DEVOUR YOUR ENTIRE SOUL!! Buh? Oh. I guess I turned into a demon again. G'morning, Mika!"

- As a demon he remembers Mika's desires before changing to anger then back to himself.

"Those 'stupid humans' are all your friends now, y'know."

- Informing Mika on Shinoa Squad's status.

"Ferid... He's the one who split the two of us up... and... killed Akane and the rest of our family."

- With Mika recalling the consequences of Ferid's actions

"Ha ha ha... I was just so overwhelmed that you were still alive and all... I guess I was too busy looking at you to see anything else."

- Saying to Mika about why he never noticed Ferid in Shinjuku.

"I guess that makes us the same!"

- Unbothered about losing his humanity and helping Mika feel better about his state of being.

"Hey, Mika? Do you think Ferid let me escape on purpose?"

- Discussing matters with Mika and considering the reality behind himself shooting Ferid and escaping Sanguinem as a child.

"This was planned. All of it. From the beginning. Then... When I got out of the vampire city... The fact that I came across Guren right away was... No...! Guren cried. I saw him crying...! Is he somehow tangled up in this whole thing, too? But whose plan is it? ...So did this plan start before the catastrophe? In that case, Kureto Hīragi can't be behind it. He sounded like he was still looking for information about it. Guren can't be behind it either. Is it Ferid? It might be worth talking to him at least once..."

- Yu's thoughts and considering a bigger picture.

"No. That wasn't your fault. Since before we were born...we've been caught up in someone else's plot. That's why every time I run, my friends and family die. I'm sick of it. This time, I'm going to try fighting fate."

- Easing Mika's guilt and deciding to change his course.

"No! What you did gave us a future! Because you found us a map! You found us a way to quit being livestock—"

- Supporting Mika's previous plan.

"No, you listen! This time, we do it together! It won't be just you. We'll both leave. We'll both get out of this screwed up world together! Let's do it again. Let's go find our map out of here. Together. And this time, all of us...will make it out of the dark alive."

- Beside Mika and not intending to have him shoulder everything by himself.

"Just now. If we're after the same thing...I don't mind allying myself with you."

- Unable to win in combat Yu presses for an alliance with Ferid

"Th-this is bad. Really bad, Mika. I... I can't hold it back... I'LL KILL YOU!!"

- To Mika and then Ferid after he loses control over his demon side when Akane's head was shown.

"Who does that bastard think he is?! First he goes and says that it's Guren who destroyed the whole world, but now he won't tell us anything else!! That snarky, smug, grinning bastard!! He needs to stop beating around the bush and tell me everything right now!!"

- Ranting about Ferid.

"Be grateful we have comrades at all."

- Yu to Kimizuki.

"I hope Mika will be okay... Look at them. They all have red eyes. Even Mika."

- From within a bus Yu observes Mika speaking with other vampires

"The rescue is already decided. There are things I need Ferid to tell me, and I owe the queen for saving my family. So we rescue them."

- Saying to Crowley on why he needs to help Ferid and Krul.

"Hrm. Hang on a sec. Just in case... Lemme calm myself first.... Akane, I'm sorry. I'm sorry I ran away all by myself. But I promise you, I'll definitely bring you back to life."

- Taking possession of Akane's head.

"I don't give a crap about the world. I can understand how Guren must've felt.""

- To Mika when Mika warns him that reviving the dead will destroy the world.

"I... I don't care what people call me. Call me a demon. Call me a monster. Whatever. It's just... If I can keep my family safe... If I can protect the people who took in someone like me, I'll be happy... Everyone here is my family."

- Concern for his squad over himself.

"Like I care. I'm going to get our family back, and I'm totally willing to do anything to do it, too. The end justifies the means! That day, I wasn't able to admit to Akane that we were family. But next time.. The next time I see her, I'll say it. If I get another chance, I'll tell her that we're family. And to get that chance, I'll do anything. Even if it means killing people, I'll do it. I'll do whatever it takes to protect my friends and my family. That's what it means to be human. Right, Mika?"

- Remaining determined Yu explains his goals to Mika.

"No. We go together. I'm sure we'll be just fine as long as we're with each other."

- Yu to Mika.

"Y'know, I don't remember much about when I was a kid, either. I wonder what's up with all this amnesia everywhere...?"

- Yu to himself

"I'm Mitsuba's family. The only thing we lay down our lives for is the rest of our family. Your stupid cause can go jump off a cliff for all I care."

- Yu to Aoi Sangū when she tells her sister Mitsuba that she has no value.

"I hear the Hīragis are supposed to be doing all kindsa super incredible, super dangerous, super evil research. Mebbe they've got something we can use that'll be all like, poof! You're human again! Just like that! And poof! I can perfectly control my demon and my angel, too! Like a magic wand! And theeeen... Maybe we'll find something that let's us do the forbidden—bringing the dead back to life. We'll bring Akane, the kids, and everyone else back. We'll do whatever it takes... And make any sacrifice we have to make. Any sacrifice."

- Hearing of and encouraged by what he knows of the Hiragi family said to Mika.

"I don't really care about my memories. I have friends now. And family! If I have them, I don't need anything else!"

- Divulging to Shikama Dōji what is important to him.

"Forget about me! Just promise me you'll find some way to keep Shinoa from turning into a vampire!"

- Being escorted away to be tested on to unlock his memories all to help Shinoa

"Mitsuba, I'll be fine. You go be there for Shinoa, okay?"

- Reassuring Mitsuba just before Yu is struck with a needle and then kidnapped by the Hyakuya Sect.

"Woooow. What a pretty girl."

- Yu from Ancient Greece when he sees Mikaela's corpse

"They say I hafta die! I'm not allowed to live! Everyone would be happier if I was dead! Because I'm demon spawn ..."

- In prison as a child, upset at what his parent's told him.

"I did find one. I have a reason to live. I have you, Mika."

- Answering Mahiru's question that he did find a reason to live as Yu takes ahold of Mika's arm.

"I... I don't want to be alone. I don't ever want to feel this bad ever again. I... I just can't do any more than I already have. I wish I could disappear. Vanish forever!"

- After Mika does not come back, Yu processes the situation.

"No matter where in the world you are, I'll find you and save you! So wait for me, okay?!"

- With Mika becoming a demon who could kill them, Yu leaves with Krul

"This is super-duper bad! She was going to teach me how to save Mika! But now she's gone!"

- His reaction to Krul's brief absence and why he needs her

"I heard him! Just now! Mika is calling for me!"

- Alerting others that he can hear Mika, from the world he awoke in after being enveloped by an explosion.

"Mika needs helps! He's calling for me! And if he needs me, I'll go to him! Wherever he is! Anybody gets in my way an' I'll run 'em over!"

- Determined to help Mika.

"My Mika."

- About Mika

"I'm gonna to save him... I'm going to save Mika... so you get out of my way. I'M GOING TO SAVE MIKA! AND YOU... ...CAN'T! STOP! ME!!"

- With three seraphs backing him, Yu speaks to Asuramaru.

"I can feel the power inside. Right now, my desires have gone completely berserk. Man. It feels so good... ...living purely for my desires. Now... let my heart shout that desire aloud... I'm saving Mika."

- Regarding his capabilities.

"Kimizuki!! Can you hear me?! It's me! Kiseki-o! Gekkouin! They're both our enemies!"

- Using Kiseki-o's coffins to contact and warn about Kimizuki and Yoichi's demons.

"Mika. I'm here. You called for me, right? Mika? Mika. It's okay, Mika. No matter what happens... no matter what's to come... The whole world could end tomorrow... ...but everything will be fine. I promise... ...that I'll save you."

- Speaking to demon Mika who is indifferent to everything occurring around him.

"Where am I? Wow, the sky's red! It almost looks like blood. Is it sunset? Mikaaa? I'm here to save you!"

- His reaction at finding himself in a new location before promptly setting about finding Mika. Yu provides a description of the sky that wouldn't be depicted in the black and white manga panels.

"Wait... I know this place. This is where Mika came back to me."

- Recognizing the store where he and Mika met again in the envisioned Nagoya he finds himself in.

"Hold on. If you'd forgotten me, you wouldn't have picked this store. You remember who I am."

- Confused at demon Mika not knowing who he is, feeling he chose the store location.

"So, Mika? I'm not gonna let you eat me. I'm gonna make you submit instead!"

- Forthrightly speaking with demon Mika as he readies his sword.

"Let my desire go berserk. I'm going to save him. I will. I'm going to! Let's go, Mika."

- Prepared to save Mika.

"Sorry, Mika. But I hafta hit you. I hafta hurt your heart. I don't wanna, but it's to save you."

- Reluctant to hurt Mika in a fight to subdue a demon.

"I want to die. How come I had to survive? Somebody kill me. I'm an ugly, worthless devil. Don't leave me. I don't wanna be alone. I wanna die. I wish I never existed."

- His inner thoughts after his family were murdered.

"Oh yeah. That's right. The deepest wound in our hearts would be this, wouldn't it? Every night I hafta have this stupid, stupid dream. But, Mika! You have to listen! I'm gonna turn you back into a human! Then I'm gonna bring Akane and all the kids back to life! There's a way to so that! So you've gotta open your eyes! REMEMBER ME!"

- Finding both his and Mika's deepest trauma is the loss of their family, and calling to him about remedying it.

"Look around you! This is the inside of your heart! We're family!"

- Trying to make Mika remember.

"But if I'm gonna save you then I hafta possess you for a sec! So let me cut you once, okay? I swear, Mika! I'm going to save you!"

- Promising to save Mika.

"Woot! Yay! Mika's room! Mika! Hey, Mika! It's time to get up, Mika!"

- A past Yu from Ancient Greece comes to Mikaela's room.

"Mika'll wake up for me! Cuz it's my job to wake him up! I was born to do it. Father told me so! So if I just call his name enough, he'll wake up! Right, Mika? You'll wake up for me, right? C'mon Mika! It's time to get up!"

- Speaking to Noya, revealing details on what the past Yu was made to do. The "father" likely being Shikama Doji, Yu tries to wake up Mikaela's corpse.

"Hey, Mika! It's me! Get up, 'kay? C'mon! There's lots of fun things to so out here! Mika! Mika Mika Mikaaa! Get up! I'm bored! Mi-"

- Trying harder to wake Mikaela up before being murdered by Noya.

"Oh! What am I? I'm... Ooh! Ooh! Ooh! Wow! I got out somehow! What's going on? Where's this? It look fun! Hey, guys! Hey, guys! Who're you? Where's this?"

- A newly created Yu excitedly notes his situation, compared to one before, and behaves as an energetic child as he speaks to Noya and Ashera nearby.

"Mika. Hey, Mika. It's me. Send your mind back to me. If we work together we can get away. Together, we can go anything. It's okay, Mika. Let's do it together. I'll devour you and you devour me. That will let you become my demon."

- Yu in the present calms demon Mika down who is under attack and encourages him to become his demon.

"Mika. Let's do it. Together we can. Mika. Close your eyes and say my name. Call me with your heart."

- Speaking to demon Mika who is being assailed by the First.

"Yep. Don't remember it at all. (to which demon Mika asks if he is an amnesiac, Yu replies) Ha ha! Yeah, that's right! I'm an amnesiac. Forgot that. Oh, but I totally remember the last 16 years. And about how I thought you'd died... ...And how I desperately tried to survive so I could get revenge for you. But then right when I find you again and think I've finally got you back... Bam! You're suddenly a little kid and don't remember the last 16 years. Ugh! Do you know how much of a pain that is?"

- Remembering nothing of the ancient past, yet detailing what he does know regarding his recent experiences with Mika.

"Yeah, but it'll still work out. I'm sure of it! We've more family besides the two of us. It's not just the two of us anymore. Don't you remember? Akane and the other kids. Then there's Guren. Shinoa. Mitsuba. Yoichi. Even Kimizuki."

- Reminding demon Mika of their family.

"Isn't that awesome? The world may be a total a total wreck... but we still have that huge of a family. There's still plenty of value to living! There's still a powerful desire... to live and protect our family. Mika. You can devour all that desire."

- Emphasizing their family, and offering the desires for Mika to devour before Yu asks him to become his demon.

"I want my friends to love me. After all... ...I can't love myself. I need to have others to do it for me."

- Needing desire, Yu thinks of one..

"Wow, cool! So you do have your memories! That's awesome! Kimizuki! Yoichi! Guess what! Mika says he got his memories back!"

- Excited that Mika as a demon remembers.

"Oh! By the way, guys! Thanks for coming to help!"

- Thanking Kimizuki and Yoichi before they return to their physical bodies.

"H-hey, it all worked out in the end, right? I survived to meet you again. And boy am I glad I did!"

- Joyful, that although being by himself was really hard, with Mika returning, Yu feels everything worked out.

"Well you had this weird, troubled look on your face. Whatcha thinking about?"

- Attentive to Mika.

"Oh, wait! I get it! You were a vampire, so you couldn't before! But now you're a demon, so you can. Ahaaaa"

- Understanding why Mika can look at the sun.

"It's the same with Asuramaru. If I make the walls around my heart reeeally high... ...Then you can't see anything anymore."

- Explaining to Mika why he couldn't see his thoughts.

"Yeah. Believe it or not, I'm competent when I want to be! I'm a grow-up now and all."

- Responding to Mika saying he is actually taking things seriously.

"Then you aren't okay?! I knew it! Guren! Shinoa says she's feeling bad!"

- Unaware that Shinoa was joking about feeling ill, Yu picks her up and calls to Guren for help.

"I've got a plan. A good one! And it'll get us to the bottom of whatever they're plotting."

- Where he trusted Guren, Yu is willing to listen and act on Mika's concerns and has an idea to find out the plot against them.

"Were you in cahoots with Ferid... ...When he showed up... ...To murder my family?"

- Upfront in asking Guren such questions.

"I said yes or no, Guren! Did you kill my family?! What about the others? Kimizuki's family?! Yoichi's family?! Did you kill them too?!"

- Furious in attacking Guren when demanding answers.

"Aha, okay! So that's why I started feeling sluggish."

- Having felt it but unaware he was poisoned until informed by Mika.

"I probably shouldn't say this... ...But I'm actually kinda excited for this. Can you feel it?"

- Sharing his excitement with Mika while battling Guren.

"It's not over yet! If we can't clash blades... ...Then I'll just hafta beat you... ...Like this!"

- Holding Guren's sword arm, Yu acts to head-butt him to win.

"I forgive you!"

- Shouting how he forgives Guren after he confirmed he murdered his family.

"You aren't alone, Guren. Take our hands!"

- Having encouraged the squad to join in, Yu speaks for Guren to do the same.

"Guren? We're family. So whatever the problem, we'll all solve it together."

- After helping Guren to his feet, Yu would seek details on his plan.

"If we're stuck living in this crummy world... ...And are all doomed to die eventually anyway... ...I wanna go out having faith in my family."

- Putting matters into perspective and sticking by who he sees as family which includes Guren.

"It'll be just like in those fairy tales you tell me... The prince rescuing the princess! I guess that makes me the prince, huh?"

- A past Yu revealing that Sika Madu reads to him, and feeling he is the prince to rescue Mikaela.

"I know! Do you think I can fix him with a kiss? There was a story like that, right?"

- Asking Sika Madu whether Mikaela would wake up if he kissed him, and also aware of a story involving that.

"Guess what, guess what! I've got a secret! Want me to tell you?"

- Excited to share a secret with Sika Madu.

"So yeah. Mika's dead, right? So he can't talk. That's why we have to bring him back to life. Right? But sometimes... ...Mika will still talk to me."

- Yu revealing something about Mikaela who is said to be deceased.

"So I put my ear to it close, just like this... ...To make sure I don't miss whatever he says next. So, I'm not bored! I'm waiting to hear him again!"

- Displaying how he listens out for Mikaela who is said to be speaking.

"What the heck?! No, really! What's wrong with you?! You get this suuuuper serious look on your face... ...And you're all like... "I'll tell you the truth.""

- While doing an impression of him, Yu presses Guren to share his plan.

"You said you'd tell us. So tell us already! Right, guys?! Don't you all agree with me?!"

- Calling to others to agree with him about Guren sharing his plan.

"You said it before, right? Your ultimate goal was to bring back everybody killed by the catastrophe. That's why you're gonna bring them all back, right? Kimizuki and Yoichi's sisters... My family. Everyone. But bringing people back from the dead isn't free. You brought back only a handful of people... ...And what happened to the world?"

- Acknowledging the price involved for human resurrection, Yu addresses how only a few came back.

"We're family! So forget the past. The future's more important! This was all some mega-plot by Shikama Doji? So what?! What do we care?! All of us are gonna fight for our family... ...And do whatever it takes, for as long as it takes!"

- A proclamation concerning their past and future.

"Okay! Let's go! Like, right now! Quick!"

- Yu keen to head into Akihabara to gain answers.

"Wait, hold on. How's this work, then? I mean, Mika's a demon now. Are Ferid and Krul gonna die too?"

- Yu seeks clarification on the resurrection.

"I don't care about humanity!"

- His response to the choice of resurrecting all of humanity and restoring Mika.

"I can't make Guren do all the hard stuff."

- Apparent misdirection when he really means to choose another option over restoring humanity.

"Hngh!! Not enough power. Need more. Lots more. Mika... Give it. Give me more power!"

- Having been thrown into the ground by Krul, his sole demonic focus is to become stronger.

"I'll devour your desires... ...And make Mikaela eat them too."

- Having looked into Asuramaru's heart, Yu shares his desires in case he did not know.

"No. You were right not to give up on your sister. That's how humanity should be. I'm not going to give up on Mikaela."

- In a demonic state Yu speaks that Asuramaru was right to help Krul, and regardless of how he feels now Yu is committed to Mika.

"Like you, I won't give up until I die... No. I won't give up even if I die."

- Similar to Asuramaru, yet resolving to never give up no matter what.

"All of them... They're there for me. I'm not alone. Not anymore."

- Talking calmly while his squad are envisioned, yet it also works to have Asuramaru lower his guard by speaking how he is back to himself.

"Pwah! Yes. I did it. I won! Boy, winning is kinda empty. Sword..."

- Succeeding in consuming Asuramaru's power, Yu quickly changes from a joyous relief to comment on the hollow nature of the victory, before moving on to contact Mika.

"Whoa, you know a lot! Do you like Tokyo Tower that much?"

- Paying attention to Mika saying that the tower is over 1,000 feet tall. Impressed, Yu also feels this means he likes it.

"I know what it was! It's called terminal velocity! Partway down, air resistance stopped me from falling any faster! So I win, Tokyo Tower!"

- Having physics based knowledge, Yu is also triumphant towards a tower after falling off it.

"You were gone! But I stayed alive and didn't die for this long! I deserve a compliment for that!"

- Crying to Mika, and emphasizing how hard it was to live after he was believed to be gone.

"Plan? What plan? I don't have a plan."

- Although he mentioned he had one, Yu immediately answers Mika the truth of the matter.

"That's why I climbed to the top of the highest place around. I wanted to get a good look at everybody we're gonna save."

- A reason provided to Mika for climbing atop Tokyo Tower.

"Yep! Think, Mika! It's gotta be easier than getting a bunch of kids away from vampires."

- Encouraging Mika to think of a plan to save everybody.

"Don't worry. I know you can do it. And if we try and it doesn't work, we'll both just die!"

- Believing in Mika, and considering an outcome in the event of failure.

"Yep! Must be a sign that I've been a good boy lately."

- Pleased to share with Mika that he found a can of food, the comment is also made after he devoured Asuramaru and left his entire squad (though they were acting against him).

"So have you come up with anything yet? Any good ideas on how to save everyone?"

- Turning to Mika to create their plan.

"But civilian supplies are distributed from the towns. If we take theirs, they'll starve."

- Knowledgable about distribution lines and concern for civilians in smaller settlements.

"Hey, you guys! Everybody, freeze! We're taking your food! Resist and I kill you! Got it?!"

- Stealing from a settlement of young people.

"Sorry. Hang in there just a little longer, okay?"

- Speaking to the girl he holds as a prisoner.

"That's the spirit. But if you don't get stronger, your sisters are gonna die anyway. This world's crap like that."

- Another instance of being pleasant but to someone he has just mistreated as he focuses on other areas.

"It's supposed to be a good hiding place too. Do you know about the big Sea of Trees at the foot of Mt. Fuji?"

- Having knowledge of the location, Yu has another reason for approaching Mt. Fuji.

"Well, yeah. We came this far. So who wouldn't?"

- Making no secret about wanting to climb Mt. Fuji.

"I get the feeling that, as my demon, some of your memories are mixed with mine. Somehow."

- Having seen the First with his disciples, Yu considers the scenario may be a mix of his and Mika's memories.

"Yeah. That's cuz he was really close then. Can you feel him now? Where is he?"

- Reasoning how Sika Madu could discern them before, and thinking to check his current condition.

"Then the First we just saw won't see us back? That was him in the past."

- Trying to ascertain whether the Sika Madu just seen is him or a memory.

"Good point. I guess if we see him again, we should run."

- Willing to flee from the First should he and Mika comes across him again.

"No. I can't. But I know I did. I was always here. Watching and talking."

- While unable to recall specifics, Yu is aware past conversations took place.

"Yeah. If this kid really is you... ...Wouldn't you remember this too?"

- Wondering if Mika does not remember speaking to a past Yu.

"What, seriously?! Then I was born in the age of dinosaurs?!"

- Registering that he has memories from prehistory.

"Hell... If this place is in my memory... ...Then doesn't that mean I have to be nearby?"

- Considering a past version of himself could be nearby.

"It's the age of the dinosaurs. At least let me be some big, cool one."

- Preferring to be a cool dinosaur rather than a small lizard.

"What era is this, again? The age of angels?"

- His question with a seraph like civilisation during the era of the dinosaurs having been discovered.

"Still, where'd he go? There're so many angels flying around I dunno if we'll find him."

- Yu wondering where Mikaela went.

"Y'know? I wonder if angels eat. Didja see any restaurants? Any ramen places?"

- Thinking whether seraphs eat having considered his own hunger.

"What's he reading? Can we read it too?"

- Questioning Mika on what the First is accessing on screens in the Tower of Babel.

"Is this really the age of the dinosaurs? It looks like we're in some way, way, waaay far-future city."

- Having glanced outside a window to the prehistoric seraph civilisation.

"No, Mika don't."

- Speaking aloud Mika's name to the memory of Mikaela as he regards a dagger.

Trivia

 * The name Yūichirō means "excellence, superiority, gentleness" (優) (yū), "one" (一) (ichi) and "son" (郎) (rō), where 「優一郎」 translates as "kind first son."
 * His parents spelled it with the kanji for "gentle" in the hopes that he would grow up to always be kind and gentle to whomever he met. Mahiru believes this idea is ludicrous for anyone less than a god.
 * Due to Yu's name, it is considered a pun when other characters say Yu is kind (優しい). Kindness is also considered one of Yu's actual primary character traits.
 * Hyakuya 「百夜」 means "hundred nights."
 * Amane 「天音」 means "heavenly sound."
 * He was one of the first characters designed, along with Mikaela and Guren.
 * Yūichirō is the only member of Shinoa's Squad who does not have a known biological sister. Shinoa has Mahiru, Yoichi has Tomoe Saotome, and Kimizuki has Mirai.
 * In the Michaela light novels, Mika asks Yu what his surname was, but Yu can no longer remember that his name was Yūichirō Amane and solely goes by Yūichirō Hyakuya.
 * When Yu first forms a contract with Asuramaru, it is revealed that 1/10th of him is not human. He becomes even less human in chapter 34 after overdosing on curse stimulant pills . At the end of chapter 40, he chooses to erase his humanity completely.
 * Since he lived in a vampire city for four years where as livestock under the vampires where he was not educated, Yu is nearly illiterate and only writes phonetically without using any Kanji.
 * Yu offers his blood three times before he finally convinces Mika to bite him in chapter 36 so the first vampire Yu is bitten by is Mika.
 * At the age of twelve, Yu was 145 cm tall and weighed 38.2 kg.
 * According to the Q&A with Takaya Kagami, Yu's seraph form has black wings because his demon tries to stop the transformation.
 * In the popularity poll, Yu earned 3rd place at 3,536 points. In the 2019 popularity poll, Yu ranked 1st with 8,886 votes.
 * According to the Manga 8.5 Fanbook:
 * Interests: Worrying about Mika / Driving cars.
 * Favorite Food: Curry, Shortcake (because the children at the orphanage said they wanted to try birthday cakes).
 * What he looks for in the opposite sex: “Shut up.”
 * According to the TV Anime 108 Fanbook:
 * It's been stated that Yu's favorite color is blue because it reminds him of the blue sky he saw once he escaped from Sanguinem.
 * Yu's favorite pajamas are yellow.
 * One of Yu's dreams is to drive a fast car.
 * Yu's goal after saving Mika is to eat curry.
 * Yu does about ten push-ups and additional training daily.
 * The past Yu in his life with Ashera is shown to already have Seraph powers, as he grew angel wings behind his back.

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Yūichirō Hyakuya