Mei Aihara is one of the central figures in her story. She is a high school student who serves as the president of her school's student council and is also the granddaughter of the institution's chairman, a position she is being groomed to eventually inherit. She is physically characterized by her straight, silky black hair and deep purple eyes, and she presents herself with a formal and composed demeanor.

Her background is marked by the abrupt departure of her father, Sho Aihara, from both her life and the academy. This event was a profound emotional wound that shaped much of her subsequent behavior. Having greatly admired her father, his abandonment left her feeling isolated and burdened. She took on the responsibility of maintaining his legacy at the school, laboring under the hope that he would one day return. Raised under the strict and demanding expectations of her grandfather, Mei learned to suppress her own desires and emotions, leading her to accept an arranged engagement with a male teacher as a matter of duty.

On the surface, Mei projects an image of a cold, stern, and ruthless individual who is a strict enforcer of the school's many rules. She is highly intelligent, studious, and carries herself with a maturity and composure that can make her seem unapproachable. However, this exterior hides a deeply sensitive, lonely, and emotionally fragile interior. Having been deprived of affection for years, she struggles profoundly to understand and properly express her own feelings, particularly those of love and vulnerability. Her typical method of communication is not through words but through physical actions; she often resorts to making abrupt and forceful sexual advances, which stem from a distorted understanding of intimacy and a misguided attempt to either assert control or find solace during moments of emotional distress.

Her primary motivation throughout much of the story is rooted in her past and her familial obligations. Her actions are largely driven by a desire to bring her father back to the academy, believing that by preserving his place and fulfilling his duties, he might return. This sense of duty to her family and the academy is a heavy chain, causing her to prioritize the school's future over her own personal happiness. She is a character who does not believe she has the right to ask for anything for herself, and she views relationships as transactional bargains rather than mutual desires.

Her role in the narrative is catalyzed by the arrival of her new stepsister, Yuzu Aihara. Their relationship begins with significant conflict, as Yuzu's impulsive, optimistic, and emotionally open nature stands in stark contrast to Mei's pessimistic, measured, and closed-off personality. Yuzu becomes the primary agent of change in Mei's life. Through Yuzu's persistent and unconditional affection, Mei is slowly forced to confront her own buried feelings and dysfunctional coping mechanisms. Over time, she begins to open up, becoming gradually less cold and more caring, though she continues to struggle with being honest about her emotions.

Key relationships in her life include her engagement to her homeroom teacher, Amamiya, which is a formal arrangement with no genuine affection on her part. Her relationship with her father is complex; while she revered him as a role model and teacher, his abandonment left deep scars. Her friendship with Himeko Momokino, her vice-president and a childhood friend, had grown distant but is later rekindled with Yuzu's help. Her interactions with the manipulative younger girl Matsuri Mizusawa are initially antagonistic, as they see a reflection of their own emotional coldness in each other, but they eventually find common ground.

Over the course of the story, Mei demonstrates a significant arc of development. She transitions from viewing Yuzu as a nuisance and a mere emotional crutch to genuinely caring for her as a partner. While she struggles to define her feelings, she becomes more confident in her identity and relationship, even accepting that not everyone will approve of their love. In later stages, she becomes gentler and warmer while retaining her calm and strict demeanor. She moves from passively accepting the futures arranged for her to actively making her own choices.

Among her notable abilities, Mei is exceptionally intelligent and an effective leader, capable of handling the heavy workload of the student council president while maintaining high academic standing. She demonstrates a quick aptitude for learning new skills, such as video games, when given the chance. However, she has notable weaknesses, including a complete lack of technological literacy—she does not own a smartphone and struggles with even basic feature phones. She is also not skilled at sewing or household tasks, often relying on Yuzu and her stepmother for domestic help. In terms of personal quirks, she has a hidden fondness for teddy bears, her ears are a sensitive spot, and she falls asleep very quickly.