Mamoru Sasaki is the protagonist of the anime Room of Guilty Pleasure, a single high school teacher burdened by an exhausting and underpaid workload. He appears physically and emotionally drained, often relying on coffee prepared with an excessive amount of sugar to get through late nights of overtime. His appearance includes dark brown hair and black eyes, and he is characterized by a perpetually tired demeanor. Sasaki's motivation is shaped by a desire to escape his mundane, stressful routine; he would rather leave work on time than find fulfillment in his job, but constant demands keep him tethered to the school. His role in the story begins when he discovers that the student shoe lockers have transformed into selection panels. Mistaking the phenomenon for a dream, he jokingly picks a student, which initiates a series of forbidden late-night encounters in a surreal space called the Room of Guilty Pleasure. This triggers a descent from a professional authority figure into a man grappling with repressed desires and moral fatigue. His key relationships are with three students: Mai Hoshino, a popular girl known for her inability to lie; Makoto Hojo, the quiet and studious class president; and Aya Hinata, the calm, easygoing eldest daughter of three siblings. Each student brings a different dynamic, challenging Sasaki's authority and often shifting the power balance through vulnerability or directness. As the series progresses, Sasaki experiences internal conflict between his professional responsibilities and the allure of the secret encounters, leading to a blurring of reality and hallucination. The encounters also build a strange trust that carries over into his waking life, deepening his guilt and self-reproach. Sasaki does not possess supernatural abilities; his defining traits are his extreme fatigue, his moral ambivalence, and his gradual loss of control as he becomes further entangled in the transactional reality of the Room.