Koji Imai is best friends with Ryōta Satō in class 5-2. He possesses a bold and quick-witted personality, coupled with a savvy understanding of what girls expect from boys. His childhood included a habit of flipping girls' skirts. He frequently engages in competitive duels with Ryōta to determine superiority in various activities. Koji harbors a distinct fear of Yūki Asano's cooking, well aware of her poor culinary skills. His physical appearance varies: black-haired in the anime, blond in the OVA. His assigned class number is 3.