Nora to Oujo to Noraneko Heart
Nora Handa is a high school boy who meets a girl named Patricia in a park who claims to be a princess of the underworld. Patricia and Nora 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.