Monochrome Factor

Akira Nikaidou is a typical slacker high school student who thinks he has a normal life until a mysterious man called Shirogane appears and tells him to meet him at the school that night. He is skeptical but goes anyway...and 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.

(Source: Tokyopop)