Miyabi Aizawa is a student at Holy Forest Academy, attending class 3-4, and serves as the primary antagonist as well as a deeply troubled figure in the story. She presents herself as a model student and class leader, but in reality she orchestrates a campaign of bullying against the teachers assigned to her class. Her actions stem from a traumatic past that destroyed her trust in adults. In the anime, her best friend Miyuki Yokoyama became pregnant by a teacher named Mr. Saito, who then abandoned her, leading to Miyuki's suicide. In the manga, the catalyst is different: Miyabi developed a crush on her junior high homeroom teacher, Mr. Ogi, who praised her beauty and intelligence. When he invited her to dinner, she believed he reciprocated her feelings, but he instead introduced his fiancée. Humiliated and feeling used, Miyabi took illicit photographs of herself and falsely accused Mr. Ogi of raping her. A classmate, Takumi Ishida, took revenge by assaulting the teacher, resulting in Ogi being fired and Takumi expelled. Overcome with guilt and rage, Miyabi turned the entire class, then known as class 2-4, against all teachers, initiating a sustained rebellion.

Miyabi's personality is defined by a seemingly diligent and composed exterior that hides intense anger, bitterness, and a merciless determination to destroy any teacher who crosses her. She becomes obsessed with getting Eikichi Onizuka fired, even more than other teachers, and often crosses ethical and legal boundaries to achieve her goal. She can be cold and manipulative, cutting off friends like Tomoko Nomura when they make mistakes. Yet beneath this hardness, she retains traces of the kind and friendly girl she once was; she is visibly saddened by Tomoko's emotional struggles, and her guilt over the harm she has caused accumulates over time. Her family life is also strained, as she knows her parents are unfaithful to each other and doubts that they love her, which deepens her cynicism toward adults.

Her role in the story is as Onizuka's principal adversary among the students. She is the mastermind behind the systematic teacher-bullying that defines class 3-4's reputation. She attempts to frame Onizuka, sabotage his teaching, and rally other students against him. However, she is also a troubled heroine who undergoes significant development. Onizuka's unconventional methods eventually break through her walls. In the anime, after she is accused of attempted murder and assault on a school board member, Onizuka takes the blame for her, and she begs his forgiveness, beginning to trust adults again. In the manga, her guilt and family issues lead her to a suicide attempt from the school roof, but Onizuka accidentally knocks her off, then leaps after her, landing on a car. After Onizuka helps resolve her family problems and she reconciles with her friends, she gradually warms to him and becomes one of the first to learn about his cerebral aneurysm, showing genuine care.

Key relationships include her former best friend Tomoko Nomura, whom she exiles from the group but eventually reconnects with; Urumi Kanzaki, an old friend who becomes a bitter rival after siding with Onizuka, leading to violent schemes on both sides; and her parents, with whom she has a broken relationship. Her most pivotal relationship is with Onizuka, who refuses to be driven away and ultimately saves her from her own despair. Notable abilities include her intelligence, leadership, and strategic planning, allowing her to coordinate elaborate schemes against teachers and manipulate her classmates. She also possesses a strong will and considerable resourcefulness, though these are often turned to destructive ends until her redemption.