Denpa-teki na Kanojo

Juu Juuzawa is a delinquent high school boy who just wants to be left alone. One day, he is approached by Ame Ochibana, who claims that she knew him in a previous life, and now wants to serve him as his "knight." At first Juu wants nothing to do with Ame, but after a classmate is murdered, he accepts her help as he looks for the killer.

(Source: ANN)