Yūichirō Hyakuya

Yūichirō Hyakuya 「百夜 優一郎, Hyakuya Yūichirō」 is the main protagonist of the Owari no Seraph series. He is a member of the Japanese Imperial Demon Army and Shinoa Hīragi's squad in the Moon Demon Company. He, along with Mikaela Hyakuya, possesses the "Seraph" gene, which is relevant to the forbidden taboo and the focal point of the story, "Seraph of the End."

Appearance
Yūichirō has medium length black hair which can be considered quite messy. He possesses sharp green eyes which fall under his thick eyebrows. His eyes are large compared to most other male characters in the series. At the beginning of the series, he is often shown wearing a typical Japanese school uniform with a white sleeved T-shirt beneath.

After joining the Imperial Demon Army, he wears the standard JIDA uniform which consists of a long-sleeved black coat with light green details including the shoulder epaulettes which has a brown Sam Browne belt 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. Yūichirō keeps his sword on the left side of his hip. Sometimes he can be seen wearing the JIDA hat, and often wears the JIDA cape as well.

Personality
He is brave, fearless and somewhat reckless, which was shown when he attacks a Four Horsemen of John while ignoring orders. It is noted not to be the first time. Yū is often called an idiot by those who know him. He is a very narrow-minded person, whose goal is to annihilate all of the vampires because of what they did to his "family," and he will overcome any type of challenge in order to accomplish it. He swore to kill all vampires because they killed Mika.

Despite this, he has a soft spot and worries about his friends first and his goals second. Ferid Bathory considers this attitude to be "cute". Because of this, he empathizes with others who put their family first, as seen when a civilian girl gave his squad false information because vampires are holding her family hostage. After learning why she deceived them, he tells the girl that she did the right thing to save her family. He feels guilty for abandoning Mika and has nightmares about Mika's death for four years until he makes friends with Shinoa and Yoichi. After learning Mika is alive, Yūichirō's personality changes in response and he grows more mature, as noted by Shinoa. He puts more effort into being patient and calm and making friends, and immediately amends his worldview to include Mika even though he is a vampire. His dream of wiping out vampires takes second place to reclaiming Mika. Regardless, even though his best friend is now a vampire, Yū still slaughters them without mercy or hesitation. After Shinjuku, he becomes closer to his squad members and makes Guren promise to help him take Mika back under the promise that Mika will be useful to him, as he comes from the same orphanage as Yū. Additionally, friendship and bonds become more important to him, as shown in a mental battle against Asuramaru, to gain more power or else lose his body, he calls Asuramaru his friend and convinces Asuramaru to yield to him.

Rather than revenge, his primary goal changes to protecting his new "family," in order to avoid a repeat of past events. In Nagoya, he surprises Narumi's squad by memorizing all of their names and promising to protect them.

Similarly to how vampires do not bother to remember human faces and only think of them as "humans" or "livestock," Yū 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. The only exception to this rule is Mikaela Hyakuya, and seems to recognize Ferid Bathory.

History
Yūichirō Amane was born in 2004. During his childhood, both of his parents screamed that he was a demon spawn. Yūichirō's father (and possibly his mother) tried to kill him. His father screamed that he had to kill the seraph of the end now or else the world would be ruined. His mother went mad and committed suicide, and Yū's home went up in flames.

Prologue
After this incident, Yū was taken to the Hyakuya Orphanage in Tokyo on Christmas Day in the year 2012, and told he could meet friends who were just as unwanted as he was. He met Mikaela and the other orphans. Seeing as the two 8-year-olds were the eldest, Mikaela wanted them to be friends, but Yū rejected him. Mika asked if Yū was challenging him to a fight and said he was the leader. They fought, and Mika gave him a serious smack-down. Afterwards, he told Yūichirō that he used to be alone like him, but now he had a big family. And from that day onward, Yū had one too. Yūichirō told him about his parents' violent behavior, saying that "family" no longer meant anything to him. Mika smiled, saying that both of his parents abused him until they got sick of him and threw him out of a moving car. The orphanage took him in after that. The other children shouted out their stories, saying that they never met their parents, their parents committed suicide, or their parents left them at the orphanage door. Mikaela then told Yūichirō that they weren't lonely because that day was the day their new big brother Yū joined them. Shortly after, the director suddenly began to bleed from her nose and mouth and collapsed to the ground.

The adults perished, and Third Progenitor Krul Tepes led the vampire invasion on Tokyo. For the next four years, the orphans survived as livestock for the vampires in the vampire capital of Sanguinem, which is located under Kyoto.

Yū and Mika encountered Ferid, who invites them to his house to partake of their blood, but Mika agrees to go while declining politely for Yū, claiming Yū to be "shy". After Mikaela steals a gun and a map of the underground, they made a daring escape attempt with their orphanage family. Unfortunately, this was all a trap set by a seventh progenitor named Ferid Bathory, who had relations with Mika. Ferid slaughtered the other children in front of Yū and Mika. Yū managed to shoot Ferid in the head, but Mika was fatally wounded. Yū called Mika "family" for the first time. He refused to leave Mika behind until Mika yelled at him and pushed him away. Yū escaped, leaving Mika behind; this action haunted him for years afterward. After reaching the outside world and realizing the vampires lied about the world's destruction, Guren Ichinose of the Japanese Imperial Demon Army finds him.

Guren mentioned a prophecy and knew that Yū was one of the lab rats from the Hyakuya laboratories. He was one of the children that was being experimented on by the Hyakuya Church to create a weapon of mass destruction, as part of the 'Seraph of the End' plan in destroying the world. Guren knew of this beforehand, thanks to intel provided by Mahiru in 2011. In order to take revenge for his family, Yū joined the Japanese Imperial Demon Army as Yūichirō Hyakuya.

Second Shibuya High Arc
Guren agrees to give Yū the power to kill vampires when they first meet, but he does not put Yū in a vampire extermination unit right away. Four years later, Yūichirō is a private of the Japanese Imperial Demon Army. He gets suspended from the army for disobeying orders during an encounter with a Horseman of the Apocalypse, and is thus forced to attend Shibuya High School #2 in the meantime. When Shinoa Hīragi tells him his suspension will continue until he makes friends, Yūichirō struggles since he has stubbornly rejected forming close relationships since Ferid Bathory slaughtered his family. He insists he does not need any friends. Shinoa gives him a note from Guren specifically written with the orders to make a friend; he promptly wads it up and throws it to the floor.

With a little persuasion from Shinoa, he steps in to save Yoichi Saotome from bullies. Unfortunately, Shinoa tells him his punishment will be extended if he harms any civilians. Yoichi tells him he wants to join the army to get revenge for his sister's death, but Yū tells him to forget the army and that his sister wouldn't want him to avenge her death. An emergency warning goes off, and Yū challenges the escaped vampire assailant, nearly killing her when Guren arrives and ends her life. Yoichi becomes Yū's friend, meaning that Yū actually reached the requirement Guren gave him. In a dream, Yū's orphanage family tells him farewell.

Yū learns that Shinoa has a demon weapon for slaying vampires and wants to take it from her by force. She rapidly relieves him of that notion. Yū, Shinoa, and Yoichi save Yamanaka's friend Yūji. During the fight, Yū makes contact with the myo-o demon but breaks out of the demon's illusion on his own, surprising Shinoa. She agrees to notify Guren.

In chapter 4, Yū meets and fights with Shihō Kimizuki before attending his first class in the Moon Demon Company Barracks. Guren introduces Yū as the idiot and Yoichi as the crybaby. Yū gets paired with Kimizuki for a practice test. When Kimizuki's sister goes into critical condition, Yū punches Kimizuki and convinces him to go see her during the middle of the exam. They become friends and rivals. Shinoa recommends both of them for the Black Demon Series.

Although Yū earns a zero on his spell craft exam, which is supposed to help determine what rank of cursed gear he can receive, Yū is permitted to try for a Black Demon Series weapon along with Yoichi and Kimizuki. He appears to be nearly illiterate since he used no Kanji on his exam and wrote everything phonetically. Yū challenges Asuramaru, earning the demon's power in order to protect everyone, and Asuramaru informs Yū that 1/10th of him is not human. Afterward, Yū talks to a possessed Yoichi and helps him to regain control over his demon Gekkōin.

Shinjuku Arc
In chapter 8, he meets Mitsuba Sangū, who completes their 5-person squad. A Four Horsemen of John chases a young girl. Against Shinoa and Mitsuba's orders, Yū rescues the girl, and a group of vampires attack him. Mitsuba slaps him for his disobedience, but he declares he does not regret his decision. In the following chapter, they use information from the girl to lead an attack against the vampires. Although the vampires seize Mitsuba, Yū's strength shines through, and he rescues her. Even though the girl deceived them, informing the squad that there were only seven vampires instead of nine vampires, Yū tells her she did the right thing to protect her family.

Before they head to Shinjuku, Kimizuki hot wires a hummer. This impresses Yū, who says he had not been in a car since he was little and anyone rarely uses cars in Shibuya since gas is such a precious commodity. Yū attempts to drive and nearly accidentally crashes into his squad members, leading Kimizuki to violently remove him from the vehicle. Kimizuki promises to teach him how to drive on another date. As they head to Shinjuku, they encounter Thirteenth Progenitor Crowley Eusford. Yū manages to take his right arm momentarily before Crowley is summoned away by Seventh Progenitor Ferid Bathory.

They reach the battlefield in Shinjuku, and Guren summons Shinoa's squad to the 5th Street intersection. After taking his supplement, Yū charges into battle. Enraged over seeing Guren getting stabbed, Yu pierces Mika through the chest with Asuramaru, calling him a bloodsucker and telling him to die. Then, he recognizes his not-quite-dead best friend and family member. Guren grasps Mika's sword with a spell tag and urges Yu to activate his weapon and kill the vampire, but Yu freezes when he realizes who he just stabbed. Mika releases his sword and escapes. Guren hits Yu and asks him why he did not kill that vampire when he had the chance.Yū says that person is Mika, his family. He cries from happiness. steps behind Yu and grabs his shoulder, causing Mika to amputate the responsible appendage. Of course, Ferid says he was just joking about tasting Yū. Mika steps up to Yu and tells him to forget about the humans and run away with him. He lifts Yu and carries him in his arms. Mika warns Yū that Yū is in a bad place and is being used by the humans, but Yu has no idea what he is talking about. Yū struggles and breaks free, jumping to the ground. He says that Mika makes it sound as though he is no longer human and asks if it is his fault. Mika immediately tells him it is not Yu's fault. Mika begs Yū to come with him, insisting that he is Yū's only true friend and the humans are only deceiving and using Yū. Yū looks back when Shinoa screams and sees his comrades defeated by the vampires. When he tries to help them, Mika grabs his wrists and says they have to go. Just then, the special supplement Shinoa gave Yu under Guren's orders takes affect, causing Yu to transform as his seraph component runs wild. Yu screams. Yū swings at Mika and Crowley before moving to kill Shinoa, his true target. Mika jumps in the way, and Yū stabs him through the back. Mika urges Yū against killing humans. He asks Shinoa how she could dare use Yū like that, and the berserk Yū calls Mika a vampire, telling him to not interfere and saying that all humans must be killed. Guren orders Shinoa to embrace Yū to undo the transformation, so she jumps in after Yū flings Mika aside. Yū cries out and hacks up blood before passing out.

Post-Shinjuku Arc
Yū wakes up exhausted a week later.

He looks up books on vampires to try and learn how to turn Mika back into a human. Later, Lieutenant General Kureto Hīragi summons Yū to the Shinjuku Central Army Headquarters First Office where Kureto sicks a starving unarmed vampire on him. Yū kills the vampire without hesitation. Kureto challenges him to a duel but cheats, winning quickly. He then attacks Kureto while his back is turned, but his aide intercepts. He also makes Major General Shinya Hīragi confront Yū. Kureto tortures Yoichi and Kimizuki to try and get information from Yū. He tells Yū about the Hyakuya Sect, asking if Yū was from that family. Yū knows nothing about them.

Guren tells Yū he knew about Yū's connection with the sect all this time, and he agrees to help Yū get Mika back. Shinoa and Mitsuba teach the squad about possession- and materialization-type demons. Yū weakens the curse binding his demon before Kimizuki and faces illusions of his orphanage family and his biological family. After Kimizuki conquers his demon as well, the two spar. Both of them use special abilities, which should be impossible for the possession-type demons they use. Yū uses a technique called "Asura-Kannon," ordering it to fan-out; this produces numerous materialized swords.

Shinoa tells Yū about what he really did on the battlefield of Shinjuku, and they agree to side with Guren in the upcoming political power struggle.

Nagoya Arc
They depart for Nagoya, and Yū drives for the first time. When Guren scolds their squad for being tardy, Yū takes the blame. Yū meets Sergeant Makoto Narumi, tricking the sergeant into believing Yū is 20 years old until Guren announces otherwise in chapter 26. Guren, Mito Jūjō, and Shinya face off against Shinoa's squad, handily giving them a one-sided beat-down before their commence their operation in Nagoya.

Shinoa and Narumi's squads team up with Shinya to exterminate Fifteenth Progenitor Lucal Wesker, and they do it successfully without any injuries to their squads.

The Moon Demon Company regroups, and Sergeant Aiko Aihara arrives with only seven survivors from her squads. Yū offers to keep watch while her squad takes a short rest. Guren Squad, Shinoa Squad, and Narumi Squad leave to hunt Thirteenth Progenitor Crowley Eusford, Seventeenth Progenitor Chess Belle, and Horn Skuld while Aiko waits for other survivors to pass them the order to reconvene at Nagoya Airport.

Once they reach Nagoya City Hall, Guren looks at the hostages being used through a telescope and knows that they are being used as bait to lure them. Yū insists on bursting in and saving them. Narumi dismisses it as too dangerous and when Yū asks him what they should do, Narumi asks Guren what they should do. Guren suggests that they snipe them from their position and see if they can lure them out but says that if there are too many of them, they are to abandon the hostages and run. Yū shouts that they can't do that.

Shinoa interrupts Yū and asks Guren what their priorities are. In order, he says that their top priority is to maintain their numbers, letting no one die. Their second priority is to free the hostages and lastly, getting the hostages out alive. Narumi states that this is because they are needed for another mission after this. Guren then says that they need to keep the vampires occupied in Aichi Prefecture for as long as they can so they can buy enough time for the main forces in Shibuya to get prepared.

After Crowley catches Gekkōin, Guren and his squad discuss what to do. After careful planning, Guren decides that his squad will take his squad to attack Crowley while Shinoa and Narumi Squads free the hostages. He gives them five minutes to rescue as many hostages as they can and act as a proper diversion for the main force in Shinjuku. As soon as the five minutes are up everyone has orders to retreat to the Nagoya Airport immediately.

Sometime later, Yoichi reports on the situation with Guren's squad to the squad and Yū insists on rescuing him. Shinoa agrees with him but says they only have five minutes. She gives them specific instructions regarding their plan.

Yū breaks through the floor and attacks Crowley. Crowley blocks easily and moves to decapitate Yū. Kimizuki parries the attack, and Mitsuba distracts Crowley by summoning dummies with Tenjiryū. Yū uses Asura Kannon, but Chess and Horn immediately step in and defend Crowley from it. Chess and Horn counterattack and rip apart the shield Mitsuba forms with Tenjiryū.

Guren orders Shinoa Squad to leave. Crowley recognizes Yū as the one who went berserk in Shinjuku.

When Yū moves to rescue Guren, who is on the ground behind Crowley, Shinya stops him and orders him to abandon Guren. Shinoa says their five minutes are up and orders a retreat.

Outside, Yū breaks free of Kimizuki's grasp and complains about leaving Guren behind. Yū asks if ignoring feelings and abandoning family is acceptable as long as it is for their mission. He holds back tears and states that would mean giving up on family. He calls Guren family.

Yū recalls four years ago after Guren picked him up from Sanguinem. One of Guren's men reported that Yū woke up screaming, and Guren comes to check up on him. Yū cries in his bed. Guren asks about what dream Yū had, and Yū says its the same one he always has: Ferid Bathory slaughtering his family. Guren asks if this means he wet the bed again, and Yū snaps, stating that he ever wet the bed. Guren says he will stay there until Yū falls asleep. He sits by Yū's bed and reads documents on the Seraph of the End project begun by the Hyakuya Sect. He stays there from 2:20 a.m. until 2:50 a.m. Yū says he has never really had a family since he was born because his parents called him demon spawn and tried to kill him. He says he made a new family, which was killed anyway. He says that living is painful and wants to know if it is even worth it.

Guren states that living hurts him, too. He says he failed to save the one he loved most, but, if he sticks around, then a day may come where someone needs him. He instructs Yū to live until he finds a new reason for life and people who need him. He leaves Yū and orders him to sleep.

In the present, Yū wipes away his tears and states he cannot follow orders. He says he is not cut out for teamwork. He tells the others to proceed without him, and he will stay behind. He says he would rather die than be unable to save his family. Before anyone can stop him, he swallows two curse stimulant pills for a total of three since the beginning of the mission. Shinoa orders him to spit them up, but Yū begins to cough blood. The curse power overwhelms him, forcing blood out of his eyes and making him scream.

Yū coughs up blood after overdosing on pills to increase his power at the end of the previous chapter. Inside his mind, Asuramaru, calling him hopeless, comments on how Yū appeared to give up on revenge and then became obsessed with living and family. He asks Yū if he wants power that badly. Yū says he does need power to save Guren. He begs Asuramaru to give him power.

Asuramaru says yes. If Yū wants power, he’ll give him all he asks for and more since he feeds on ugly human emotions like obsession and greed. However he complains that the pills Yū took before are a problem as they force the curse to go crazy while keeping his greed in check. Asuramaru says that the contradiction won’t do. He states that Yū cannot expect to just take drugs and get stronger without any consequences so Yū asks how he can get stronger.

Asuramaru says he can’t answer that as Yū is already dead. He says that it was a quick death. Yū took so much of the drug that all his internal organs ruptured and his heart exploded. He says while he has powerful regenerative abilities, but that kind of damage is beyond him. He says Yū’s done. Dead. Then, he amends his statement, saying that at least he should be.

The only reason he isn’t is because of those disgusting angels. Their power is forcibly keeping him alive. They peer into the background of Yū's mind and see something lying on the ground. Yū asks Asuramaru what it is. He says he doesn’t know but from there it looks like a trumpet. Yū has the urge to take the trumpet, but Asuramaru warns him not to take it and insists that Yū take him instead since angels can be far nastier than demons.

Asuramaru notices that Yū’s heart has starting beating again and says that now that it’s fixed, he can take care of the rest. He stabs into Yū's abdomen, telling him not to go with the trumpeters and instead stay with him, accept him. He tells Yū that he can heal him but when he tries to he says that he cannot return Yū to normal. He complains that Yū lost even more of his humanity than before and has become even uglier. Asuramaru orders him to never do that again and claims Yū's body is his. Yū says he does not care and demands for Asuramaru's power. Asuramaru calls him a greedy little human wretch and tells Yū that he has already given it to him as three pills worth of the drug is in effect. He can use all his strength for the next two to three minutes without becoming a demon. Asuramaru then tells Yū to awaken.

Yū wakes up to his friends' panicked faces. He says he is okay and has something to do. With immense speed and power, he leaps from the ground and breaks through the wall to reach Crowley and attacks him. He demands that Crowley give Guren back, and Crowley wonders if he is really human. The two fight, and Crowley delights in the fact he may have to use both hands for once.

Guren, whose wrists have been bound, gets between them and orders Yu to leave. When Yū asks why he is still not strong enough, Crowley answers that it is because Yu is only livestock and asks if Yu fears cows and pigs revolting someday. He insists vampires will never fear humans. Yū repeatedly yells at him to shut up and die, and they exchange blows. Guren orders Yū to stop. He says that Yu's demon will take over if he uses more power than that. Yū refuses to leave.

Crowley mutters that he is beginning to get bored and decides to kill Yu. Guren tells Yu that he has a plan, but Yū needs to leave. Yū does not listens and demands even more power from Asuramaru.

Asuramaru is delighted to help, especially now that the protection from the drug has completely worn away. Yū's desires overwhelm him, and half of his face darkens and a horn sprouts from his head as he becomes a demon. Yu enjoys the power. Crowley says that Ferid always makes interesting things happen. He says that Yu is so lively and full of energy that he wants Yū's blood now and mentions he may drink Yu dry the next time he gets close.

Yu challenges Crowley again. He hears telling Kiseki-Ō to begin the count to nine until the devil's coffin opens. After Kiseki-Ō gets to four, Yu invites Kimizuki to kill this bloodsucker together, but Kimizuki notices Yū's horn. Guren warns Kimizuki that Yu has been possessed and to grab him and run. Yū tells Guren to shut up now that he has the power to rescue him.

Kimizuki tells Yū that that what he said before was right. In this screwed up world, there's no point to living if you don't have a family to cling to. He then says that's why it's his turn to rescue him. Kiseki-Ō finishes the countdown and captures Yū with the devil's coffin.

Kimizuki joins with the rest of their squad while Kiseki-Ō begs to kill Yū, who is in his coffin. Kimizuki orders him to only knock out the demon inside of Yū.He orders Kiseki-Ō to spit him out and catches Yū in his arms.

As Mika engages the Moon Demon Company, he sees Yū being carried by Kimizuki. Yoichi recognizes Mika and asks his group if that vampire is Yū's family.

Shinoa calls out to Mika to identify him, but he tells her to shut up and return Yū to him. Shiho Kimizuki says the should stop him before Mika kills them. Yoichi protests. Mika reaches for Yu. Mitsuba tries to block him, but Mika smacks her axe away. Kimizuki drops Yu while trying to block Mika and says Mika cannot have Yū. Mika throws him aside. Mika begins to lift Yu from the ground, but Yoichi jumps on his back. He says he is not letting Mika go anywhere, and Mika tells him to quit getting in his way. Yoichi says Mika can kill him, but Mika is their family so long as he is Yuichiro's family. Mika knocks him off as well, telling him to not be stupid and to die.

Shinoa Hiragi activates Shikama Dōji and knocks Shinya away from Mika, surprising both of them. With her back to Mika, she explains that Yū would be mad at them if Mika were to die. She orders Mika to take Yū and run.

Mika comments that family does not exist in this messed-up world. He recalls playing with his own family on the streets of Sanguinem and pulls Yu close to him. Shinoa tells Mika to hurry up and then meet up with them at Nagoya Airport. Mika asks her why he would meet up with them, but she says she decided to trust him and that is that. Her squad charges at the Moon Demon Company while Mika flees with Yu.

While escaping, Mika holds the unconscious Yu in his arms and pleads for him to wake up.

Natural Abilities
Yūichirō seems to be an excellent swordsman, as he was trained by Guren himself. This is shown when he killed a Horseman of John using only a sword.

He is also a "Seraph of the End" hybrid with 1/10th of him being not human; he and his old family were experimented on by the Hyakuya Sect, though he doesn't recall it because the adults erased his memories after each experiment. After he overdoses on the curse supplement after vowing to save Guren from Crowley's merciless hands, his non-human part increases.

Cursed Gear

 * Asuramaru 「阿修羅丸, Ashūramaru, lit. "Perfect Asura"」: Asuramaru is a possession-type demon weapon of the Black Demon Series. It resides inside a black sword that Yūichirō uses. Whilst in use, the blade develops a thick green aura. Like Shinoa 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.

Experimentation

 * Drugging Up: Like other soldiers in the JIDA, Yū 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, and officers only resort to this as necessary.
 * Seraph of the End: By swallowing a special tablet produced by Guren, Yū can lose his mind and undergo a painful transformation. His left eye turns dark and bleeds while a massive wing-like structure sprouts from his upper left shoulder. Although extremely powerful, he goes berserk and attempts to kill any human nearby.
 * The Progenitor Council is extremely concerned with his immense power in this form. Guren causes this transformation to end by making Shinoa hug Yū. Yū coughs up blood as the transformation ceases. The drug takes five days to get out of Yū's system, and he spends the total of one week in a coma. Even after that, he feels tired. Yū has no recollection of the events that occur while he is transformed.
 * Anime: The transformation is more extensive. Yu's left eye eventually turns into a black pit, and his right eye becomes red.  In the vampire alone, he attacks both vampires and humans without discrimination while he only wants to kill humans in the anime and orders vampires like Mika to get out of his way.  Additionally, his power is demonstrated more in the anime by even swinging his sword and creating a plume of black energy the size of a tall building with a ring of light around its base.

Trivia

 * When Yu first forms a contract with Asuramaru, it is revealed that 1/10th of Yu is not human.
 * Since he lived in a vampire city for 4 years, Yū is nearly illiterate and only writes phonetically without using any Kanji. Vampires have no need to educate livestock on how to read or write.
 * Yūichirō 「優一郎」 translates as "kind first son."
 * 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 is one of the first characters designed, along with Mikaela and Guren.
 * At the age of twelve, his body height was 145 cm and had a weight of 38.2 kg.
 * According to the fanbook:
 * Interests: Finding things about human turned vampires to help Mika (Worrying) / Driving vehicles
 * 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.”

Quotes

 * "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!"--12-year-old Yū to Mika in the streets of Sanguinem, Chapter 1, "The World of Blood Legacy"
 * "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!"--8-year-old Yū to Mika the day Yū arrives to the orphanage, Chapter 1, "The World of Blood Legacy"
 * "Quit trying to shoulder all our burdens by yourself!"--12-year-old Yū to Mika in Sanguinem, Chapter 1 "The World of Blood Legacy"
 * "You're... my family. I... finally found a real family! I can't leave you!"--12-year-old Yū to a fatally injured Mika, Chapter 1, "The World of Blood Legacy"
 * "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!!"--Yū to a myo-o demon, Chapter 3, "The Demon in Your Heart"
 * "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."--Yū to Shihō Kimizuki when told that Mirai Kimizuki is in critical condition, Chapter 4, "The Worst Pair"
 * "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."--Yū before first challenging Asuramaru, Chapter 6, "Black Asura"
 * "DAMN IT!! SHUT UP, SHUT UP, SHUT UP!! Just give me your power already!! I... need it!! I need enough power to protect everyone!!"--Yū confessing the truth of his feelings while confronting Asuramaru, Chapter 6, "Black Asura"
 * "I don't ever wanna say I survived because I left someone else to die ever again!!"--Yū to Shinoa's squad when he enters an obvious trap to save a human girl, Chapter 8, "Mitsuba's Squad"
 * "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!"--Yū to the enemy vampires in Omotesando Station, Chapter 9, "First Extermination"
 * "...'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..."--Yū to Mika when they reunite, Chapter 13, "Reunion of Childhood Friends"
 * "S-s-ssi-sinners... S-sinners must... d-die. M-must kill... all s-si-sinners."--Berserk Yū preparing to kill Shinoa, Chapter 13, "Reunion of Childhood Friends"
 * "Do not interfere, Vampire. All humans must be killed."--Berserk Yū to Mika when Mika protects Shinoa, Chapter 14, "Everyone's a Sinner"
 * "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!"--Yū to himself, after waking from a coma, Chapter 15, "Complicated Connections"
 * "Just you wait, Mika. I swear... I'll save you."--Yū to himself, Chapter 15, "Complicated Connections"
 * "...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."--Yū to Shinoa when asked what he was looking at, Chapter 18, "Possessed by Mahiru"
 * "I tried climbing back up using my sword, but some stupid janitor yelled at me for putting holes in the walls!"--Yū after returning from the roof Guren previously flung him from, Chapter 18, "Possessed by Mahiru"
 * "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 Hiragi 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!"--Yū to Guren, Chapter 18, "Possessed by Mahiru"
 * "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."--Yū to Asuramaru, Chapter 20, "The Demon's Nightmare"
 * "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."--Yū to Asuramaru, Chapter 20, "The Demon's Nightmare"
 * "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."--Yū to Shinoa's squad, Chapter 23, "Ambition in the Demon Army"
 * "LET ME GO!! I said leggo!! What's wrong with you guys?! You can't be serious about leaving Guren behind?!"--Yū after Guren Squad and Shinoa Squad abandon Guren, Chapter 33, "Demon's Lullaby"
 * "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!! (Pause) 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."--Yū to the Shinoa and Guren Squads before overdosing on curse stimulants, Chapter 33, "Demon's Lullaby"
 * "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?"--12-year-old Yū to Guren in a flashback, Chapter 33, "Demon's Lullaby"
 * "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, Chapter 34, "Asura's Power"
 * "Bwah? Aah..!  So... much... power..."--Yu after becoming possessed, Chapter 34, "Asura's Power"