Yuuichirou Hyakuya

The protagonist of the manga. His nickname is "Yuu." His former surname is "Amane." He is a member of the Moon Demon Company, Shinoa Hiiragi Squad, with the rank of Special Private Second Class. Born on October 16, he is 16 years old. He is the most successful experiment of the "Seraph of the End" and the host of the Second Trumpet, known as the "Second Trumpet Blower." His demon weapon is a katana named "Asuramaru."

While he is a key figure in various plans, his birth, existence, and past are shrouded in mystery. Yuuichirou himself has no memory of his past due to the memory manipulation by the Hyakuya Sect. It is revealed by the Sixth Trumpet Blower that he is neither a demon, human, nor angel, but an "existence itself that is taboo."

At the age of 8, he was persecuted by his parents, who called him a "child of the devil," and was taken in by the Hyakuya Orphanage on Christmas Day. Shortly after, the world collapsed, and he, along with the other children from the orphanage, was captured by vampires and transported to an underground city. In the underground city, he and Mikaela took care of the younger children while planning their escape to the surface. During the escape, he lost Mikaela and the others, leading him to seek power for revenge against the vampires.

Yuuichirou has a short-tempered and straightforward personality, often speaking in a sharp tone. However, he is brave and unafraid to confront anyone, and he deeply cares for his friends and family. Initially, due to his past of being abandoned by his parents and his desire for revenge against the vampires, he intentionally avoided interacting with others and lacked cooperation. However, after joining the Moon Demon Company, he began to trust his comrades absolutely and started to show his true personality. Despite being called a "fool" by his comrades, he is trusted and admired, with Shinoa and Mitsuba showing affection towards him. Although he calls Guren "stupid Guren" and often rebels against him, he feels indebted to him and continues to follow him even after learning that Guren was the one who caused the world's collapse. He cares deeply for Mikaela, who survived as a vampire, and when he saw Mikaela in a bloodthirsty state, he gave him his own blood, turning him into a true vampire.

Yuuichirou has high combat abilities, having been trained in swordsmanship by Guren and being a holder of a Black Demon Series weapon. He is considered "a bit strong" among the Shinoa Squad and Narumi Squad by Lucal. However, he struggles against Crowley and is forced to retreat.

As one of the test subjects of the Hyakuya Sect's "Seraph of the End," when he took the specially formulated drug given by Guren, his left eye turned black, he bled, and ominous wings sprouted from his back. During this time, his personality was taken over by the Seraph, and he acted on the Seraph's "hatred towards humans," slaughtering humans regardless of his own will or memories. He possesses combat and healing abilities that surpass those of demons, playing a crucial role in battles against the Seraph of the End and the Fifth Progenitor. When his heart burst and he died instantly after taking the special drug, he was forcibly revived. When he awakened voluntarily at Nagoya Airport, he was taken over by the Seraph and tried to kill all humans after defeating the Fifth Trumpet, but he regained his self-control with Asuramaru's intervention and stopped his rampage by turning into a demon with his demon weapon. However, this caused him to oscillate between his own personality and that of the demon. Even after leaving the army following the incident at Nagoya Airport, the symptoms continued but were suppressed due to the drugs administered by Ferid. Initially, it was impossible for him to use the Seraph of the End while retaining his self-control, but after attempting to control it during the mission to rescue Krul and Ferid and defeat Luc, he managed to use it to some extent with Asuramaru's help. However, his stance of siding with humans, forgetting his role as a Seraph, is considered "broken" by the Sixth Trumpet Blower.

He bears a striking resemblance to a boy named "Yuu" who acted alongside the First Progenitor over a thousand years ago. He is called "Yuu" and seen as the same person by Shikama Doji and No-Ya. Shikama Doji speaks to him as if they are reuniting and claims to know a past that Yuuichirou himself does not. The Hyakuya Sect calls him a "demon," and No-Ya makes statements suggesting that Yuuichirou is an artificial human created by Shikama Doji, saying things like "It's the first time I've seen a properly grown Yuu" and "Did Shikama Doji succeed in creating an artificial human?"

After returning to the Imperial Demon Army, Yuuichirou allows Shikama Doji to invade his mental world, leading to Shinoa's transformation into a vampire. To prevent Shinoa's transformation, he offers himself as a test subject to Kureto. While unconscious due to the drugs, he is captured along with Asuramaru by members of the Hyakuya Sect disguised as military researchers and reunites with Guren, who has become a demon. During this time, he is rescued by Mikaela and, along with Mikaela and Asuramaru, fights to save Guren, who has become a demon. However, Mikaela is mortally wounded, leading to his transformation into a Black Demon. After Mikaela's transformation, Yuuichirou joins forces with Shinoa Squad and Krul, trying to find a way to save Mikaela. During this time, Krul reveals the origin of the current plan. When Shikama Doji, who has taken over Shinoa, intervenes, Krul reveals that Shikama Doji is the mastermind. Guren then appears and tells Yuuichirou to turn Mikaela into his demon weapon, and he begins the ritual.