Buso Renkin Vol 3 (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!

Never underestimate the power of underwear! With no battle gear to speak of besides his butterfly mask and skivvies, Papillon Mask survives a bitter duel against our boy Kazuki. Meanwhile, Papillon Mask's great-great-grandfather Dr. Butterfly promises him the true powers of a humanoid homunculus, which includes the ability to use a Buso Renkin. Will Kazuki and Tokiko be able to fend off their powerful - and eccentric - nemesis?