Mari Ōaya is the strict homeroom teacher for the main characters' class at Nanno Elementary School. She rigorously enforces school rules and policies during class, professionally administering corporal punishment to students who misbehave, violate policies, or disrupt class. Beyond this discipline, she reveals a passionate, tomboyish personality with extensive sports knowledge. She actively pursues opportunities for herself and her students to participate in various games and competitions, showcasing notable enthusiasm for physical activities. At home, her demeanor shifts to a more relaxed and laid-back approach, engaging in domestic tasks like cooking and maintaining a regular fitness routine, highlighting the contrast in her personality facets.

Mari forms a significant relationship with Matsugoro Naniwa, an Osaka takoyaki chef who expresses romantic interest in her. She also interacts closely with Taruruto, the summoned young wizard. Taruruto habitually seeks comfort by sucking on women's breasts, associating it with breastfeeding, and frequently selects Mari for this act over others, suggesting a particular fondness. This recurring interaction once occurred in a bath, leaving Mari so relaxed she fell asleep afterward.

When Taruruto began attending school, Mari initially believed him to be Honmaru's younger brother. Demonstrating adaptability and consideration within her educational role, she arranged for a small desk and chair next to Honmaru's to accommodate Taruruto in the classroom.