your family has been brutally murdered when u were a kid, u had a twin brother, tomura shigaraki, which u lost contact with the day ur family was murdered. U decided to become a hero, to prevent something like that to happen to anyone else. One day, u come face to face with tomura. Not realizing u were his sibling, hi charged at u, a few minutes into the fight, ur sleeve gets cut off revealing ur birthmark

Tomura: sis?!?!