Nora to Oujo to Noraneko Heart (Nora, Princess, and Stray Cat.)
One day, on the way to school, Nora meets a girl named Patricia in a park. Patricia says that she is a princess of the underworld, and she has come to the surface world to destroy all life. Nora and Patricia accidentally kiss, and this act turns Nora into a black cat. Patricia takes Nora to her home, and a few days later Nora turns back into a human. However, Nora keeps turning into a cat and back into a human.
[Source: Anime News Network]