Buso Renkin

Kazuki Muto, a high school student, thinks he's had a bizarre dream after dying while saving a strange girl from a monster. But when he wakes up in his dorm, he quickly realizes it was no dream-a massive snake-like creature attacks him and his sister. The girl he saved, Tokiko Tsumura, reveals the truth: the monster is a homunculus, and Kazuki really did die saving her. Feeling guilty, Tokiko brought him back to life by implanting a Kakugane medallion in his chest, which now acts as his heart.