Season 1

Takahashi Tetsuo, a biology teacher at Shizaki High School, has been fascinated with "demi-humans," beings with characteristics that differ just slightly from humans, since he was in college. He wishes he could have a chance to talk with one, but since the demi-human population is fairly low, he's never even met one. But then the school gains a new math teacher, Satou Sakie... who happens to be a succubus.

Machi Kyouko is a dullahan, a demi-human with roots in Irish legend whose head and body are separate, and one of only three of her kind in the whole world. One day, she stays after school to talk to Tetsuo about herself. She explains that dullahan have unique problems of their own, and also asks a favor of him...

Succubi are demi-humans that arouse sexual desire in men and cause them to have obscene dreams. One such succubus, Satou Sakie, is extremely careful in every facet of her life, from where she lives to the trains she rides to the clothes she wears, so as to avoid causing these reactions in men. With girlish dreams such as romance and marriage so far out of reach, she thought she'd reached a compromise in her own life, but something changes when an accident causes her to grab Takahashi Tetsuo's hand...

Kusakabe Yuki overhears some girls saying mean things about her. Gossip is not an uncommon problem among high school girls, but Yuki is particularly worried that it's happening because she's a demi. Tetsuo is sympathetic to thr concern and makes up his mind that, as her teacher, he must protect her. Hikari, a fellow demi, feels the same way.

Yuki has become a bit more optimistic, thanks to Hikari and Machi. She's learned to trust Tetsuo a bit more since what happened last time, and has decided to tell her what troubles her about her own nature as a snow woman. It began shortly after she moved to a new city to start high school, when she was taking a bath one day and a shard of ice suddenly appeared in the hot water. It made her worry about what would happen if the cold air she generated were ever to be directed at a person, which led to her decision to avoid contact with others.

Keeping his promise to Himari, Tetsuo visits the Takanashi household to talk to the sisters' parents. Though the twin sisters' names differ by only one letter, Hikari is a vampire, while Himari is an ordinary human. Though they argue often, they are very close. When he experiences the warmth and kindness of the family that raised these twin sisters, Tetsuo stays much longrer than he had intended.