Chikyuu Kanrinin

A girl named Nanoha survived a plane crash fifteen years ago when her heart was replaced with one from the Great Demon Maou, allowing her to live after her mother's death. Now, Maou has returned, demanding the heart back. To avoid this, Nanoha moves into Mezon Aasu, a residence for magical beings where Maou resides. She remains in possession of his heart for now, but must manage the Earth Mansion despite the presence of its original owner.