Yuuichirou Hyakuya is a member of Japanese Imperial Demon Army's Demon Moon Squad and the main protagonist of the Owari no Seraph series. He was abandoned by his parents since he was a child and he ended up at Hyakuya Orphanage. Since then, the kids of this orphanage were his only family. A bit after while he was still a child, the vampires took him and the rest of the orphanage to their city to become "livestock." After Michaela gets possession of a map of the underground, both of them decide to escape the place with the rest of the children of the orphanage. However it is revealed that is was all a plan of Felid Bathory, one the Seven Founder, and the attempt to escape is frustrated leading to death of all the family except for him. After getting to the outside world he is found by Guren Ichinose, a captain of the Japanese Imperial Demon Army. In order to take revenge for his family, he decides to join the Japanese Imperial Demon Army to gain the power he needs. (Source: Owari no Seraph Wikia)

Fai's introduction is in chapter two of the manga, in which he is shown sealing away King Ashura of Celes country at the bottom of a pool. He transforms his creation Chii (originally from Chobits) into a net-like shield, instructing her to alert him if King Ashura should awake. He then uses his magic to transport himself through dimensions, to the shop of the Space-Time Witch, Yuuko Ichihara. Out of his travel companions, Fai’s motives for joining the group are the least clear. He was the only one to arrive at Yuuko's shop purposefully, by use of his own means. His wish was to never return to Celes country, but as the price for this would have been too great, he travels worlds with the group instead. He fears King Ashura’s awakening, believing King Ashura will chase him through different worlds. The basis for this fear is as yet unknown. According to CLAMP, the correct pronunciation of his name would rhyme with "eye", so instead of spelling it like "Fay", as they intended, they thought that this would confuse readers. Thus, they stuck with the spelling of "Fai". In Horitsuba Gakuen, he is the chemistry teacher.