Busou Renkin

A teenager named Kazuki Muto attempts to rescue a girl from a monstrous creature and dies in the process. The following day, he discovers the events were real, not a dream. The girl, Tokiko Tsumura, was trying to kill a homunculus, a shapeshifting creature that feeds on humans, but Kazuki interfered. To bring him back to life, Tokiko implants a "kakugane" in his chest, an alchemical device that enables him to summon a lance to fight such creatures. Tokiko is part of the Renkin Kenshi, a group dedicated to protecting the world from these beings. Kazuki soon joins her in the effort to eliminate the entity responsible for creating and controlling the homunculus.