Gleipnir

Shuuichi Kagaya is a high school student with an unusual condition: he transforms into a monster without warning. He prefers to keep this ability private, but his secret is exposed when he saves a girl, Claire Aoki, from a burning building. After discovering his true form, Claire blackmails Shuuichi into telling her more about monsters. Claire's motivation stems from her search for her sister, who also became a monster, and Shuuichi agrees to help her find her.