Yūichirō Hyakuya

Yūichirō Hyakuya (百夜 優一郎), birth name Yūichirō Amane (天音 優一郎), is the main protagonist of the Seraph of the End: Vampire Reign manga. He is a member of the Japanese Imperial Demon Army as well as Shinoa Hīragi's squad in the Moon Demon Company. He, along with Mikaela Hyakuya and a few others, possesses the "seraph" gene, which is relevant to the forbidden taboo and the focal point of the story, "Seraph of the End.

He also appears in prequel materials Seraph of the End: Guren Ichinose: Catastrophe at Sixteen and Seraph of the End: The Story of Vampire Michaela.

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 male characters in the series.

As 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, he is shown wearing his school uniform, gakuran 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, 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. 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 become scarlet, sprouts two black horns from his head, and his teeth become pointed while curse marks appear on the side of his face.

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.
 * Anime: In his incomplete seraph form, the transformation is more extensive. Yu's left eye eventually turns into a black pit, and his right eye becomes red. Then in his full seraph form, Yu's wings are brown cord-like limbs and the edges of his hair do not become white.

Human
Up until his teenage years, Yu started off as 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, yet he still held out hope of finding something good in this world. At first, Yu wanted nothing to do with Mika and the other Hyakuya orphans and even challenged Mika to a fight.

Shortly after though, when Mika puts Yu in charge of watching over the kids alongside him and Akane, Yu took on the role of the aloof, yet kindhearted older brother. After losing them, Yu reverted back to being a moody antisocial rebel with a thirst for revenge against the vampires that killed his family for another four years.

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, ignoring orders. It's made clear that this wasn't the first time. Yu is often called an idiot by those who know him. He is a very narrow-minded person, whose only goal is to annihilate all vampires because of what they did to his "family." He swore to exterminate all vampires for killing his family, especially Mika. Yu will take on any type of challenge in order to accomplish his goal.

Despite this, he has a soft spot, worrying more about his friends first than his goals. (Ferid Bathory considers this attitude to be "cute"). As a result, Yu 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 becomes friends with Shinoa and Yoichi. 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 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 reclaim Mika. Regardless, even though his best friend is now a vampire, Yu 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 Yu. 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," 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. The only exception to this rule is Mikaela Hyakuya, who he still greatly cares for, and as Ferid Bathory was the one who murdered his fellow orphans, Yu still cleanly remembers his resentment at Ferid and has been seen to rage when they meet again.

Aside from this, Yu shows to have a somewhat weird sense of humor, as shown when he teased Shinoa for her height.

1,000 years ago he was in good terms with Ashera Tepes and been with Sika Madu for a long time even obeying his orders, despite being used by Sika Madu all along.

In the Dark Ages of Greece, he expresses curiosity over minor or major things such as wanting to see the "Mikaela" corpse, even when very young his personality is similarity to the present one. According to Noya, that even one or more Yu's is killed, Sika Madu would just create dozens of him, upon witnessing the current Yu in the present time Noya is surprised to see a fully grown Yu, wondering if Sika Madu succeeded in finishing the human creations.

Seraph of the End
During his first transformation, he 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.

Upon his second transformation, he curses the tainted humans and plans to turn them into "pillars of salt."

During the Dark Ages of Greece, he possessed Seraph wings for flight, and also displays the same murderous behavior as the current Yu when being the Seraph of the End.

Namanari
In Yu's demon form, he acts like a standard demon. He wants to kill and destroy anyone nearby and screams and rages about being restrained. He even attempts to persuade Mika with his blood and promises to run away with him if only Mika will set him free.

Thanks to Shinoa and Mitsuba's wrist restraints, he has been unable to act on his desires.

When he returns to human form, he does not recall any of his actions as a demon. Despite this, he appears utterly nonplussed about losing his humanity, much to Mika's distress.

Until he receives medication for keeping the demon side restrained, he acts colder about everything, and Asuramaru notes he is able to think much more rationally without all of those emotions in the way. After receiving the medication, his personality returns to how it was before he became a namanari.

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. Yet another testament of his skills is that with just a regular blade, he was able to go up against Shinoa using Shikama Doji, 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, pressuring her when she blocked his attempt on Kureto's life.

He 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. After he overdoses on the curse supplement to save Guren from Crowley, his non-human part increases.

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

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, Noin Teta.

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 millions of thousands swords surface from the warped space which materializes with a homing effect at the target. It has enough destructive force to shatter the surrounding area in the warped space.

Experimentation

 * Cursed Gear Stimulants: Like other soldiers in the JIDA, 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 ones internal organs exploding, and officers only resort to this as necessary.
 * Seraph of the End: By swallowing a special tablet produced by Guren, Yu can lose his mind and undergo a painful transformation. Although extremely powerful, he goes berserk and attempts to kill any human nearby. Guren causes this transformation to end by making Shinoa hug Yu. The transformation ceases, and puts Yu into a coma for a week. 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.

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.

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.

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.
 * 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 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, he was 145 cm tall and weighed 38.2 kg.
 * According to the Q&A with 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 it is shown that Yu already has Seraph powers as he grew angel wings behind his back.