Main characters

Seine Miyazaki (宮崎星音, Miyazaki Seine)

Voiced by: Haruka Tomatsu (Drama CD)

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 bugs, 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. Her weakness is her inability to swim. 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 Shimon, but she is slowly warming up to Tatsuya.

Tatsuya Tsugawa (津川達哉, Tsuwaga Tatsuya)

Voiced by: Mamiko Noto (Drama CD)

After the death of his parents in a car accident, Tatsuya tries to live to fulfill his father's last wish of becoming a "high caliber" man. After "saving" Seine from bullying, he insisted on trying to help her even after finding out the real goal of Seine. He is kind, optimistic and believes in other people's good side which is sometimes an obstacle in Seine's revenge. He is a bad cook, but Seine somehow is not bothered by his food. He has a crush on Seine and wants to be acknowledged as a "high caliber" man by her.

Nagisa Yoshiyuki

Nagisa is a girl in Tatsuya's class, a long time acquaintance, and would probably like to get closer to Tatsuya, but Nagisa has had violent allergic reactions whenever she is in Tatsuya's house ever since Tatsuya found a lost puppy (soon to be Crokette). She stays away from the house and the dog, effectively putting some distance between him and her, even though she loves him. She is the fourth person shown in the story to be infected by a "mushi".

Minato-kun

Minato-kun is a boy in Tatsuya's school, friend to both Tatsuya and Nagisa. Nagisa asks him to keep her allergy to dogs confidential.

Shizuki (Pistis Sophia)

Shizuki is the older sister of the "Akuma Twins". Being the older, she is sarcastic, lazy, and smart, but she has a weak body; she can get sick and tired easily, cannot swim in pools as a result of the chlorine, and almost was killed by Tatsuya's food. She has an immortal body and has lived for centuries. She gave immortal bodies to Seine and Shimon. Her former name was "Pistis Sophia" but she changed because it was "stupid". She is a Oyaji-con (she is attracted to "handsome" grown up men). The name Pistis Sophia is a character from an earlier work of Yuna Kagesaki's: Chibi Vampire, who is actually their younger sister Psyche. Shizuki and her sister call Seine "Sehne".

Yuzuki

Yuzuki is Shizuki's younger sister. She is the opposite side of her sister: her body has superhuman strength, she is diligent and a good cooker but is a little dumb. She is good-hearted and satisfies her Onee-sama's orders. She also has an immortal body and has lived for centuries. She is a Lolicon (she get excited by children, especially girls) to the point to kissing every children on their mouth upon seeing them.

Shimon

Shimon was Seine's third foster father and the one person who showed her kindness when she was young. He was known as a magician in his time and was also known to be 1700 years old (about 300 years ago in present time of the story). He bore the "Aion" mark, just like Seine does at the present time, but he had it across the right shoulder and was able to use Aion's power for his own, unknown, reasons. Seine is bent on revenge after the mermaid ate Shimon, but after Ariel said that they would "return" Shimon to Seine she just spit onto Seine's forehead, causing her to go into a coma-like sleep.

Minagi

Tatsuya's first cousin, once-removed, an elementary school aged girl. Her parents were the second and third ones shown in the story to be infected by a "mushi".

Tsukasa

Tsugawa family lawyer, responsible for managing Tatsuya's money until he gets old enough. Tatsuya calls him his "brother", which he explains as foster-brother since Tsukasa was in the house so much in earlier years. When he was first seen, he had been knocked unconscious by the Akuma twins and took a long time to regain consciousness.

Creatures of Sea

"Mushis"

"Mushi" is the name of any kind of mind controlling bug. They infect humans by touch or bite. An infected human has a normal life until they see Seine's face, then they develop an irrepressible rage toward her and a desire to kill her. Seine usually refrains from destroying the mushi before the infected human kills her once, otherwise the desire to kill her remains even after the mushi is removed.

Mermaids

They are Seine's prey. They are 7 in all. As of the start of the story, Seine believes she has killed one of them. Up to chapter 32, five of them have been introduced and named.

Sheila

The youngest of the mermaids; she is despised by all her older sisters except for Ariel. Seine thought she had killed her some 300 years before, but Ariel restored her body almost entirely, leaving her badly scarred about the neck and unable to talk, but with the ability to change herself to have legs and walk about on land. She was introduced as a new transfer student; and Tatsuya is given the assignment of teaching her sign language. Her true goal is infect a lot of people to order to take revenge on Seine for leaving her without her voice. She falls in love with Tatsuya.

Anoshu

The fourth of the mermaids. A bit of a dimwit, and not entirely a bad sort.

Enati

The third of the mermaids. Possesses Simon's skull, and supposedly all his knowledge.

Bella

The second of the mermaids. She is sadistic, heartless and cruel. She is respected (feared) by her younger sisters, with Sheila fearing her the most. The only one she fears is her older sister Ariel.

Ariel

The first mermaid and the eldest of them all. Ariel is also biggest of them and the most powerful. Only being overcoming for her "Father". She is the only one who is nice to Sheila and gave her a new body to replace the one that she had lost. She admits to loving Sheila more than she loves her other sisters.