Gleipnir

Shuuichi Kagaya, an average high school student, has the unusual ability to transform into a monster. The cause and nature of this condition are unknown to him, and he prefers to keep it hidden from others. During a fire at an abandoned building, Shuuichi uses his powers to rescue a girl named Claire Aoki.

Claire soon discovers Shuuichi's secret and blackmails him into revealing his knowledge of monsters, which is limited. However, Claire has her own motivations, seeking her missing sister who has become a monster.