Mezameru to Itoko o Mamoru Bishoujo Kenshi ni Natteita

One day Ryou wakes up as a girl, which comes as a shock for him. 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. Now, oni are gathering to make an attack on Yume, and Ryou must use his newfound powers to hold them back.