Apparently his family's main branch has long protected the secrets of oni mastering. Some of oni are displeased with this slavery and continuously target a member of the family with the highest Renki power, called the Oni-Pleasing Princess. The current Princess is Yume, Ryou's cousin and his secret crush.

Oni-Pleasing Princess receives a protector swordswoman, called the Oni-Cutting Princess. The protectors have always been female, and as Yume unconsciously wanted Ryou to protect her, he has awakened as Oni-Cutting Princess, at the same time transforming into a girl.