Buso Renkin Vol 2 (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!

With less than a week to go before the homunculus lodged in Tokiko's waist reaches her brain, Kazuki and Tokiko search for the creator of the dreaded homunculi. But little do they know that the evil genius behind the human flesh-eating monsters is an eccentric, terminally ill student at Kazuki's school!