Mari Ōaya is the homeroom teacher for Honmaru Edojo, Taruruto, and their classmates. She commands authority in the classroom with a strict and disciplined presence, rigorously enforcing school rules. She administers corporal punishment to students who break policies, cause disruptions, or misbehave.

Outside her authoritative role, she exhibits a passionate and energetic personality. She boasts extensive sports knowledge and enthusiasm, eagerly organizing and participating in athletic competitions and games with her students. This energetic streak contrasts her professional strictness.

Beyond school, she sheds formality for a more relaxed demeanor. She cooks and maintains fitness through exercise, revealing a multifaceted personality distinct from her teaching identity.

Her personal life includes navigating a romantic subplot with Matsugoro Naniwa, a takoyaki chef from Osaka who expresses affection for her. This relationship adds a personal dimension outside her educational duties.