Shinobu Matsuda

Shinobu is, in the beginning, featured as one of Satsuki's classmates. She seems to know black magic, as she teaches it to the other girls of the class so they can grant their wishes. However, the spells are actually a trap for the girls to have their souls stolen by the darkness, and Shinobu in reality is the Avatar of an evil deity called Yamime. In the end Satsuki learns that Shinobu was once her mother's classmate who mysteriously disappeared. Possibly upon her "second death", she was forever engulfed by Yamime. She re-appears in the 20th episode.

(Source: Wikipedia)