Gleipnir

Shuuichi Kagaya is a normal high school student, but sometimes he has an extraordinary ability to turn into a monster. One night, he finds a building on fire with a girl trapped inside, so he turns into a monster to carry her to safety. The next day, the girl he saved, Claire Aoki, finds him and reveals she knows his secret. Claire also has a secret of her own, she is looking for her sister who has also become a monster. To get the answers they need, they team up together on a mysterious adventure.