Reika Kitami

The mysterious academy nurse and the main antagonist of the series. She seems to take a liking for Minase, and knows about his experiments with black magic. She was used as a sacrificial virgin in a ritual for the original Witchcraft Group (which was initially led by Miss Takashiro). However, since she was in fact not a virgin (she had been previously gang-raped), the ritual went horribly awry. When the gates of Hell opened, she made a deal with Satan that kept her alive in exchange for her soul. Now, the time that Satan gave her is running out, and she is searching for a new virginal female body to use as a reincarnation.

(Source: Wikipedia, edited)