Denpa teki na Kanojo (OAV) [OVA]

Juuzawa Juu is a delinquent high school boy who just wants to be left alone. One day, he is approached by Ochibana Ame, 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-
http://myanimelist.net/anime/5177/Denpa_Teki_na_Kanojo/#9vQZ5KJrfZ1xHmiV.99