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 chidren 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 Glen 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)