Natsuhiko Moriaki teaches mathematics and serves as a homeroom instructor at Oya High School. He considers one of his students, Hotori Arashiyama, one of his two primary nemeses, fueled by her consistent mathematical failures and disinterest despite competence in other subjects. This antagonism connects to his own childhood experience where public humiliation by his elementary school mathematics teacher established his first nemesis.

Moriaki embodies a rigid, logic-driven approach to life, marked by an intense pursuit of mathematical truth and precision. This manifested early; as a second-grader, he experienced distress over division remainders, perceiving them as unresolved imperfections. Professionally detached, he earnestly strives to impart knowledge, though Hotori's persistent struggles occasionally trigger self-doubt about his teaching efficacy. His absolute commitment to principle is evident when he responded to a female student's love confession with a mathematical equation, prioritizing factual correctness over emotional engagement despite the resulting controversy.

Despite his usual inflexibility, Moriaki demonstrates a capacity to seek alternative viewpoints, consulting Hotori about enigmatic paintings left by his late grandfather and acknowledging her distinct perspective despite his general assessment of her limitations. Physically, he has grey hair, grey eyes, stands 1.76 meters tall, possesses blood type A, and was born on November 29.