Monochrome Factor Series by Kairi Sorano

Akira Nikaido 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... moreAkira Nikaido 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. From KAILI SORANO comes a rebel without a cause--who has been given the ultimate supernatural power! less

The fact that Kengo, Aya and many other ordinary people are gaining the ability to see shins proves that the serious disruption between the worlds is getting worse. And now a serial killer is on the loose! As Shirogane and Akira pursue him, they not only discover that he may be a shin, but also that a sinister power from the shadow world is about to make its move...

Akira demands answers of Shirogane, and for once he seems willing to provide them. He reveals that everything in the light world has a twin in the shadow world. A Shin's counterpart is called a Rei. And as it happens, Akira is Shirogane's counterpart, destining him to become king of the Rei. However, his transformation into a Shin has made it impossible for him to become a Rei and fully awaken as king.

Akira Nikaido's life takes a bizarre turn when he agrees to help restore the balance between the human world and the shadow world, which has gone awry, by becoming a shin, a kind of shadow creature.