Mangaupdates:
Onigawara Yuutarou is your average high school kid, a bit on the quiet, kind-hearted side. One day, he walks through an arcane ritual and ends up summoning a demon, Slayer, who will grant him 3 wishes. Horrified by Slayer's violence, Yuutarou wishes for Slayer to stop using his demon powers, turning him into a human. Now Slayer's stuck with Yuutarou in a perfect personality mismatch, and to top things off, if one of them gets hurt, so does the other.