Buso Renkin Vol 7 (Manga)

Talk about being at the wrong place at the wrong time! High School student Kazuki Muto had no clue what he was in for when he rescued damsel in distress Tokiko from a monster known as a "homunculus." Disguised as humans - who actually eat humans - homunculi are malevolent creatures that affix themselves to people's brains, and once fully grown, the only thing that can annihilate them is a weapon called Buso Renkin!

Kazuki's black kakugane, which powers his Buso Renkin weapon and keeps him alive, will soon advance to the point where it begins draining life from others around him. Teaming up with his partner Tokiko, Kazuki leaves on a mission to find out more about the black kakugane in hopes of keeping this from happening. Meanwhile they encounter an elite group of Alchemist Warriors that have been assigned to hunt them down and kill them!