Yuichiro Hyakuya, born Yuichiro Amane, endured profound childhood trauma when his parents tried to kill him, deeming him a demonic threat. His father claimed it was necessary to avert global destruction; his mother took her own life after failing to burn him alive. Rescued by Saito of the Brotherhood of a Thousand Nights, he underwent memory-erasing experiments before placement in the Hyakuya Orphanage. Initially rejecting the idea of family due to his past, Yuichiro eventually formed deep bonds with Mikaela Hyakuya and the other orphans, embracing them as his true family. Their planned escape from the vampires' underground city ended in tragedy: Ferid Bathory slaughtered the orphans. Mikaela sacrificed himself for Yuichiro's escape, though Mikaela was later turned into a vampire by Krul Tepes. Yuichiro alone reached the surface, found by Guren Ichinose. Consumed by vengeance, Yuichiro joined the Japanese Imperial Demon Army four years later. He entered the Moon Demon Company and was assigned to Shinoa Hiiragi's squad. His early personality was marked by impulsiveness, recklessness, and a single-minded obsession with destroying vampires. Initially wielding a standard katana, he later contracted with the demon Asuramaru, gaining a Cursed Gear sword. This contract exposed that one-tenth of his being was non-human, later identified as seraph heritage. Reuniting with Mikaela during a Shinjuku vampire attack shifted Yuichiro's focus. While he continued ruthless combat against vampires, his primary goal became rescuing Mikaela, fostering greater emotional maturity and patience. He memorized human comrades' names and vowed to protect them, reflecting his redefined sense of family. During the Nagoya mission, he partially transformed into a demon to save Guren, ignoring personal peril. He later let Mikaela drink his blood to save Mikaela's life, accepting Mikaela's vampiric nature. Yuichiro possesses latent seraph abilities linked to his identity as the "King of Salt." Under stress, these powers trigger physical transformations: his left eye darkens and bleeds, wing-like structures emerge, and he gains the power to crystallize targets into salt. His full seraph form manifests black wings, gold irises, and white-tipped hair, often inducing berserk states where he indiscriminately attacks humans deemed "sinners." His demon form, activated by removing Asuramaru's restraints, features red eyes, horns, curse marks, and violent impulses requiring external suppression. Historical ties reveal Yuichiro originated as a synthetic being created by Sika Madu (later Shikama Doji) in ancient Greece. Clones of Yuichiro were designed to resurrect Mikaela, with whom they shared an intrinsic bond. Experiments involved repeated decapitation and regeneration; Yuichiro's blood turned Ashera (later Asuramaru) into the first vampire. These events confirm his non-human biology and predetermined role in Shikama Doji's plans. After Mikaela's death and subsequent transformation into a demon during a battle with Guren, Yuichiro sought to save him by merging their beings, making Mikaela his demon. This act enabled mutual body possession. Confronted with Guren's plan to resurrect humanity at the cost of Mikaela's existence, Yuichiro chose Mikaela over the world, betraying his squad. He resolved to find a third option: saving both Mikaela and humanity, or dying alongside Mikaela. Yuichiro's evolution takes him from a vengeful orphan to a protector prioritizing familial bonds. His defiance of authority, willingness to become a demon for others, and ultimate choice to ally with Mikaela against all opposition underscore his loyalty. Yet, his seraph and demon traits create persistent instability; his physical transformations embody the duality of his humanity and engineered purpose.