The Testament of Sister New Devil(新妹魔王の契約者)

Basara Toujou met his two cute little sisters, Mio Naruse and Maria Naruse. The two sisters revealed themselves as a member of the demon clan and Basara as a member of hero clan, demon clan's enemy, and is not related by family. Mio is the future Demon Lord and since Mio's power is not fully awakened, Basara will protect her from the demons who are after the power she inherited from her deceased father, Demon Lord Wilbert.