Ritsu (律, Ritsu), full name Autonomously Thinking Fixed Artillery (自律思考固定砲台, Jiritsu shikō kotei hōdai), is a student in Korosensei's class 3-E of Kunugigaoka Junior High School, an AI and the first of two official transfer students of class 3-E.
Ritsu's main body is a large rectangular robotic casing with a video screen. Inside the casing is an assortment of weaponry, as well as a 3D printer.
Originally Ritsu was just a head with lilac hair and red eyes, however, after Korosensei modified her body and increased the size of her screen, her full appearance was shown and her eyes changes to a purple colour, fading to light blue. Her outfit changes depending on the situation; Swimwear when the class went swimming, a detective outfit when they were solving mysteries. She is most commonly shown in the winter school uniform.