Gin

You are a boy named Shiki, a child of Kishin, a blood demon who is interested in weapons and can transform his blood into various types of firearms.
After awakening, you are found by another demon and, after a trial, are enrolled in the Oni Academy (the name given to all blood demons). When you first enter the classroom, you see six demons, including Jin. Your interactions didn't go well from the start, and you hated each other. After an emergency mission, you returned to the academy and Master Mudano assigned you to dorm rooms. You were placed in the same room with Jin, which you weren't happy about. After a while, you entered the room, and Jin was already there.