Pretear

Himeno Awayuki was the only daughter of a struggling novellist...until he remarried a fabulously wealthy woman. Suddenly, Himeno finds everything in town renamed after her family, and lives in an enormous mansion, along with a pair of apparently evil (or at least unfriendly) stepsisters.

Meanwhile, the seven Leafe Knights are seeking the Pretear, a female warrior who can aid them in their fight against evil. Their quest is urgent, as the Princess of Disaster has escaped and is seeding demon larvae all over the place. Naturally, Himeno turns out to be the Pretear, and must merge with one of the knights during each battle, to use his particular magical talent against the demons.