"The Testament of Sister New Devil" is a Japanese light novel series written by Tetsuto Uesu. It follows the story of Basara Toujou, a high school student who unexpectedly becomes the protector of two demon girls, Mio Naruse and Maria Naruse. The series blends elements of fantasy, action, and romance as Basara navigates his new responsibilities while dealing with various supernatural threats. The narrative explores themes of loyalty, power, and the complexities of relationships. As Basara trains to harness his abilities, he faces challenges from rival factions, including other demons and heroes. The series has been adapted into manga and anime, expanding its reach and popularity among fans.