Buso Renkin

The series is about teenaged boy Kazuki Muto who is accidentally killed during a battle between an alchemy warrior and a homunculus at his high school, and after being revived through alchemy, becomes one of the alchemy warriors. These warriors use numbered alchemial devices called Kakugane that, when activated, take on a unique Buso Renkin (武装錬金, Busō Renkin?, lit. "Armament Alchemy") form based on the Kakugane's user. According to Watsuki's author notes in the manga, he based the designs of several characters on those of characters from Rurouni Kenshin.