オセロ

There are dangerous monsters called D's, lurking in the shadows of the world. A D mimics its victim's form exactly, allowing it to kill its victim and take its place. Kei, a normal high school kid, learns of these creatures when one of them targets his childhood friend Hinata. In his effort to protect Hinata, Kei finds himself being dragged into the affairs of Othello, an organization formed to bring about the extinction of these creatures. (Source: MangaHelpers)