Mai Ookubo, granddaughter of Kin'gyoku Academy headmistress Ran Ookubo, became an orphan in childhood following her parents' death. Raised solely by her grandmother in Tokyo, her life is deeply rooted within the school environment long before the main narrative unfolds.

She sports boyish short hair contrasting a gentle and kind demeanor. Initial interactions reveal her repulsion towards coarse manners and excessive eating habits, particularly those exhibited by the newly arrived Igano Kabamaru; she openly compares his behavior unfavorably to that of a monkey, highlighting her strong initial aversion.

Mai functions as the primary voice of reason among the central cast. She consistently expresses concern over classmates' dangerous or immature actions, advocating for non-violent conflict resolution. This role as the pragmatic counterpoint to more eccentric peers remains steady. She maintains an optimistic outlook, frequently smiling and attempting to mediate disputes peacefully.

Emotionally, she initially carries a crush on charismatic student council president Shizune Mejiro, unaware his attentiveness stems from her connection to her grandmother rather than genuine interest. Concurrently, she remains oblivious to romantic attention from multiple individuals, including Kabamaru, Shuu Maejima, and Derek.

Significant development occurs during a school excursion to the Iga Mountains. Caught in a blizzard and separated from the group, Mai is rescued and cared for by Kabamaru. This experience, especially his display of tenderness during the storm, initiates a shift in her feelings, allowing her to perceive qualities beneath his rough exterior.

Later, she attempts personal intervention in the escalating rivalry between Kin'gyoku and Ōgyoku schools by approaching Shuu Maejima directly. Mistaken for a participant in espionage, she is taken hostage by Maejima's associates. This incident, partly orchestrated by Hayate Kirino to provoke Kabamaru, results in her rescue by him.

By the narrative's conclusion, her feelings for Kabamaru fully develop into reciprocation. She acknowledges his positive traits and willingly accepts his affection. They make a mutual promise to reunite before his departure for further training, resolving her character's romantic arc.