From MangaHelpers:

Tachibana Karin may look like a teenage girl, but she's actually a black cat spirit. She has no memory of her own life before the day she was rescued by Mitsuya Keiji, the head of a powerful onmyouji family. Now Keiji, Karin, and a fox spirit that serves Keiji's family all attend a high school that is protected by his power. Karin longs to be useful to Keiji as a spirit servant, but she is only allowed to watch the others and learn. For their part, most of the other spirit servants think that a strange, amnesiac cat like her should not be allowed to stay at Keiji's side. What secrets are lurking in Karin's past, and will she ever be allowed to serve Keiji, as she desires? [tethydust]