Nora to Oujo to Noraneko Heart

One day, on the way to school, Nora meets a girl in the park named Patricia. She states that she is a princess of the underworld, come to the surface to destroy all life. However the first time she touches someone in the surface, she faints, and Nora finds himself nursing her. Patricia asks Nora to tell him the secrets of life, and Nora starts by explaining the adult magazine that they found in the park.

(Source: Anime News Network, edited)