Mitsuba Marui is the eldest of the Marui triplets, a trio of identical sisters who attend Kamohashi Elementary School and are enrolled in class 6-3. She presents herself as the most popular and respected student in the entire school, demanding that her classmates address her as Mitsuba-sama and treat her like royalty. In reality, she is frequently described as a precocious sadist who delights in causing mischief and dominating others. Her personality combines a grandiose sense of superiority with a tsundere streak, meaning she often denies her true feelings and hides any vulnerability behind a facade of arrogance. She is gluttonous and constantly tries to monitor her weight, though she rarely succeeds in controlling her appetite. This lack of restraint leads to frequent, often humiliating injuries, making her a frequent victim of slapstick misfortune despite her self-proclaimed importance.

Mitsuba’s central motivation is the pursuit of admiration and control. She craves recognition as the dominant figure of her class and attempts to establish a pecking order where she is served and respected. However, her schemes regularly backfire, exposing her immaturity and need for validation. Within the story she serves as both an instigator of chaos and a source of comedic suffering, often at the hands of her own sisters or the unwitting mischief she creates. Her relationship with her immediate family is a complex mix of sibling rivalry and reluctant affection. She bickers constantly with her middle sister, Futaba, whose athletic and lecherous nature contrasts with Mitsuba’s vain pretensions, and she frequently falls victim to the quiet, manipulative pranks of her youngest sister, Hitoha. The triplets’ dynamic is central to the series, with Mitsuba playing the role of the would-be ringleader whose plans rarely succeed.

Beyond her family, Mitsuba interacts with classmates such as Miku Sugisaki, a girl who outwardly opposes her but secretly harbors an obsessive crush, and her teacher, Satoshi Yabe, who bears the brunt of her troublemaking. She shows little genuine development throughout the series, remaining consistently stuck in her self-centered and sadistic behavior, though rare moments of embarrassment or kindness peek through. Notable abilities include her talent for devising elaborate harassments, her skill at manipulating situations to make others serve her, and her stubborn refusal to back down even when faced with overwhelming humiliation. In terms of appearance, she typically wears a skirt, striped thigh-high socks, a t-shirt, and a jacket, all of which contribute to her deliberately stylish, self-proclaimed popular image.