Monochrome Factor

The story revolves around high school student Akira Nikaido, a typical slacker living a normal life. That is, until he meets the mysterious Shirogane, a man who suddenly appears and tells him that they have a destiny together. When Akira hears this, he is shocked and doesn't believe a word of it. Aya, a friend of Akira, forgets something in the school one night, and asks Akira to help her and go find it. He agrees, and while there, he gets attacked by a shadow monster. Shirogane convinces him that the balance between the human world and the shadow world has been distorted and that Akira must become a "shin"- a creature of the shadow world- in order to help restore the balance.-wikipedia