Busô Renkin

The story begins when high school student Kazuki Muto is killed one night saving a mysterious girl from a monster, only to wake up in his school dorm, believing it to have been a dream; however, he soon finds out that dream wasn't a dream at all when a giant serpentine monster attacks him and his sister. Tokiko Tsumura, the girl he saved, explains that the monster is a homunculus. (myanimelist.net)