Seine is a girl who is over 300 years old due to her immortality. She became immortal by Shizuki, the older sister of the "Akuma Twins". Seine also gained the ability to summon her black snake, AiON, from inside her. AiON takes out the evil inside a person by taking out "mushi", bugs mermaids use to control people. In the first volume, she posed as a normal student in Tatsuya Tsugawa's school to find people with "mushi". She said that she's done this several times in different places.
She has a huge hatred for mermaids ever since she witnessed them devouring her father figure, Shimon, the magician (previous holder of AiON).