Ysley is a Idaten who was born about 100 years ago as a result of a natural outbreak. He can be said to be the brains of the Idaten side, and prefers to study rather than fight, but because he was naturally born, he began research to "learn about himself."

He attempted to train under Rin, but quit only two-three months after starting as he found her training to be brutal torture. He likes to study about past human civilizations and ruins. After a magical being appeared, he was pierced in the chest and defeated by it, but Hayato managed to defeat the being. Realizing that more demons will appear, Ysley decided to renew his training under Rin.