The studious, bespectacled classmate of Rin Kokonoe and Kuro Kagami's in class 4-1 (originally 3-1) at Futatsubashi Elementary School, who has been close friends with both of them since they were in first grade. Quiet and shy by nature, Mimi Usa's reluctance to speak her mind was initially mistaken for aloofness by her classmates, resulting in their distancing themselves from her through no real fault of her own. This eventually led to her being grouped together with the two other "class outcasts," Kuro and Rin, during a physical education class, an episode which marked the beginning of a mutual association that would ultimately give rise to an intimate friendship.

Despite being the tallest, smartest, and most physically developed of the three girls, Mimi's knowledge of adult and sexual matters is, ironically, inversely proportional to her level of dominance in these areas: an astonishing degree of her childhood innocence has managed to survive her friendship with Rin and Kuro intact, and the clash of her wide-eyed naïveté with the sheer volume of sexual innuendo she is constantly bombarded with, intentionally or unintentionally, often results in confusion, shock, misinterpretation, embarrassment, or some combination of all four. While Daisuke Aoki has done his best to protect Mimi's purity of mind from Rin and Kuro's impromptu "sexual education" lessons, even her two best friends have, on occasion, stepped in to shield her from some of the world's seedier realities, such as lolicons, sexual harassment, lewd commentary, and underwear fetishes. To her credit, however, while Kuro's numerous attempts to confess her love have fallen on deaf ears when it comes to the object of her affection, the existence of her lesbian attraction to Rin has registered with Mimi, who occasionally offers her friend moral support — or at least sympathy — when difficult circumstances touching upon her unrequited love arise.

Mimi's biggest problem, however, comes in the form of her struggle to find her own voice and establish a sense of self-identity. While her shyness, lack of assertiveness, and reluctance to stand up for herself make her a prime target for teasing and bullying, her father was always there to defend her until a work transfer necessitated his living far away from his family, resulting in the loss of one of her few outlets for making herself heard. Her mother's preoccupation with Mimi's older brother, with whom she does not get along, and indifference (or at least inconsideration) towards her have made things worse, as she has often found herself standing alone in the face of such challenges as dealing with the verbal abuse of her old homeroom teacher, Nakamura, and purchasing her first bra on her own after suffering much physical and social discomfort. While Rin, Kuro, and Aoki have been quick to rise to her protection and are unyielding in their emotional and moral support, since most of Mimi's defining characteristics emphasize qualities about herself with which she is either uncomfortable or that make her stand out — such as her intelligence, height, lack of aptitude when it comes to sports and related activities, and unusually generous physical endowment for a third grader — she has become incredibly self-conscious, emotionally fragile, and increasingly depressed over time.

Having fallen in love with Rin's cousin, Reiji Kokonoe, at first sight, Mimi received a significant boost in morale when he, recognizing much of who he once was in her, advised her to never lose hope that she would one day find someone who would understand and love her and encouraged her to not succumb to anger and hatred at the world like he did. Though prompted by Rin a short time later into confessing her secret crush on him — which Mimi tearfully did in spite of her fear about adversely affecting their relationship — she was surprised when her friend gave her permission to follow her heart with her blessing. Despite her deep feelings for Reiji, however, Mimi is far from blind: she has noticed subtle signs of his unusual emotional attachment to Rin, and an earlier incident — during which she saw Rin in an uncharacteriscally vulnerable, emotionally distraught state just before the two of them shared an unusual moment of non-sexual physical intimacy — have raised her concerns about her friend's continued well-being.

Source: Wikipedia