A third year Kaijou Academy student. He is Mari's brother and the person who introduced her to the world of quizzes, although he stopped competing in quizzes in preparation for university exams. However, he seems to hate the very thing he introduced to his own sister, who is now very enthusiastic about it. It was later revealed during the Asagaoka Quiz Bowl Arc that Seiji was Gakuto Sasajima's rival and teammate from Kaijou. He quit from the quiz club partly in disappointment for Gakuto leaving Kaijou to build a new one from scratch.