Miyazaki seine is the main protagonist in the manga. She is an immortal (316 years old) girl
who Tatsuya invites to live in his home; before that she lived in a cardboard box in a park. In the first chapter, Seine looks like a masochist because she likes being bullied. In truth, she becomes close to people who are infected by "mushis" (a kind of parasitic, mind-controlling bugs), in order to hunt them. Her hunting way s go against the kind and pacifist Tatsuya's beliefs. Her motive is to avenge Shimon's (her foster father) death by killing the mermaids from the sea. She controls a black snake like thing called "Aion" that lives inside her. Her weakness is her inability to swim. In the early chapters, Tatsuya calls her "Miyazaki-san". After she moved in, she told him to call her "Seine" because it sounds closer to her real name; means that "Miyazaki" is just made up and she does not really react to it when other people call her that.
Seine was born in America in the 17th century. Many years ago, when she was still 4 years old, her biological father sold her to another man who did many terrible things to her, e.g. trying to drown her, beating her with a stick, ordering her to steal some goods for him etc. she loves (or has a father complex for) a man named Simon who adopted her when she was still 4 years old. Simon then ask Seine if she wants to come with him (adopted her). Seine decides to go with him and he became her "foster" father.
Seine looks like the average15 year old in her outer appearance. She has seaweed coloured hair with yellow eyes. As Nagisa (Tatsuya's childhood friend) mentioned, and is shown that she is quite skinny. Besides the uniform she wears in school, she is mostly seen wearing Tatsuya's vests and underwear at home. Tatsuya (who is by this embarrassed) sees this as a big problem and keeps telling her to stop.
Tatsuya Tsugawa :
Seine was "saved" by Tatsuya while she was being bullied by a classmate. Since they'd met not long after Tatsuya had lost his parents, when the truth was revealed that his guardians wanted his father's inheritance money, she promises to stay with him until he has a family again. It is unclear if she has any feelings for him. Shimon:
Seine's thirde guardian. When she meets Shimon he saves her from her abusive second parent and they stay together for ten years. She witnesses Shimon eaten alive by the mermaids, and vowes to kill them all in revenge.
"Akuma" Twins :
Shizuki- She is the one who gave both Shimon and Seine immortal bodies. It seems that Seine cannot get along with her; sometimes calling Shizuki a "demon". Seine sometimes calls her a pervert.
Yuzuki- When Seine is younger, Yuzuki kissed Seine on the lips when they first met. She kept on chasing Seine until she cried (she is probably affraid of Yuzuki at that time). Seine once warned Tatsuya's cousin to not go in the house yet (since Yuzuki is interested in young children), but it was too late since Yuzuki already kissed her. After that, Seine saved Tatsuya's cousin by bashing Yuzuki's head.
- Seine enjoys sleeping in boxes
- It has seemed she has killed two mermaids