Busou Renkin

When Kazuki Mutou sees a mysterious girl under attack by a monster, he rushes to help and is killed in her place. He later wakes as though the incident were a nightmare, until the girl, Tokiko Tsumura, tells him the truth: the attacker was a homunculus, and she restored his life by implanting an alchemical device called a Kakugane in his chest as a replacement heart.

A Kakugane can transform into a weapon shaped by its user's instincts, and Kazuki's becomes a massive lance known as Sunlight Heart. Together with Tokiko, he enters a hidden war against homunculi that disguise themselves among humans and prey on them. What began as an impulsive attempt to save a stranger leaves Kazuki responsible for a new power and for protecting his family and friends from a world he never knew existed.