Miyazaki Seine is a 300-year-old girl with an immortal body who Tatsuya invites to live in his home. Before that she lived in a cardboard box in a park. In the first episode, Seine looks like a masochist because she seems to like being bullied. Actually, she becomes close to people who are infected by "mushis", a kind of parasitic, mind-controlling bug, in order to hunt them. Her hunting ways go against the kind and pacifist Tatsuya's beliefs. Her motive is to avenge her foster father Shimon's death by killing the people of sea, the mermaids. She controls a black snake called Aion that lives inside her. Tatsuya at first calls her "Miyazaki-san." She tells Tatsuya to call her "Seine" since "Miyazaki" is just made up. "Seine" merely sounds most like her real name. She claims to only love Shi