Buso Renkin Vol 4 (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 inadvertently reveals his identity as an Alchemist Warrior to the Hayasaka twins, members of the L.X.E. (League of Extraordinary Elects), when he activates his Buso Renkin in front of them. After Tokiko and Kazuki take on the twins in a late-night battle, they learn the location of the L.X.E. headquarters, and head there along with Captain Bravo. Intending to put an end to Dr. Butterfly's organization, they discover that yet another evil plan has been set in motion...