Mayumi Tanaka is a supporting character in Prison School and a first-year student at Hachimitsu Academy. She belongs to the same class as Chiyo Kurihara and Kiyoshi Fujino and is a member of the school's Go Club. Physically, she is a slim and short teenage girl with a boycut hairstyle, usually seen wearing the standard female school uniform, and in her dorm room she often wears a white shirt.

Mayumi is a friendly person who shows genuine concern for those close to her, particularly her roommate and best friend, Chiyo Kurihara. She is notably non-confrontational and avoids fights whenever possible, which leads her to rely on Chiyo for physical defense and protection. In return, Chiyo depends on Mayumi for advice and emotional support, making their relationship mutually supportive. Though she does not act hostile toward the boys, Mayumi is largely apathetic about Chiyo's interest in Kiyoshi and the presence of the five first-year boys at the school. Her personal tastes differ from Chiyo's: while Chiyo loves sumo wrestling, Mayumi prefers fashion, clothes, and magazines, though Chiyo remains unaware of this disinterest.

Mayumi's motivations are not driven by grand ambitions but by a desire to maintain a comfortable, peaceful daily life and to support her friend. She avoids conflict and prefers to stay out of trouble. Over the course of the story, a notable secret is revealed: she operates an online streaming persona under the name Makami Nayuta, presenting herself as an aspiring voice actor and idol. This hidden activity comes to light during the cavalry battle event, where she participates on the side of the Underground Student Council. When her secret is exposed, she initially struggles but is encouraged by the entire student body, allowing her to overcome the moment.

In terms of development, Mayumi's role remains largely as a supporting figure, but her hidden online identity adds a layer of complexity to her otherwise simple character. Her key relationships center on Chiyo, as her closest confidante and protector, and she interacts with other characters mainly as part of the larger school events. She does not display any supernatural or combat abilities, but her notable skill is her capacity for maintaining a dual identity through her online streaming activities.