Aoi Kajiya is one of the twin protagonists of the story and is the older brother of Akane Kajiya. As the male twin, he is established early on as a character who stands in stark contrast to his sister. Where Akane is energetic and impulsive, Aoi is defined by his calm and composed demeanor. This quiet nature is a core aspect of his personality, making him the character who most struggles to express his feelings outwardly.

Aoi is exceptionally gifted academically, a fact that becomes a central point of comparison between him and his sister. Initially, for reasons not extensively detailed, he becomes dull and refuses to attend school. This period of withdrawal is broken by an intervention from Akane, who urges him to return to his studies. Once he resumes attending school, his natural intellectual abilities surface dramatically, and he consistently holds the top rank in exams from that point onward. His academic success is not portrayed as a struggle but as an innate capability, which further fuels the rivalry and feelings of inferiority that Akane experiences.

Aoi’s primary role in the story is intrinsically linked to his relationship with his twin sister. He is the benchmark against which Akane measures herself, as she is constantly frustrated by being compared to him negatively and finds it nearly impossible to surpass him in any field. This dynamic creates a love-hate relationship that serves as the engine for much of the plot. Akane views Aoi as both a beloved family member and her "archenemy," declaring that she must surpass him in every way. Aoi, in contrast, seems to accept his abilities without arrogance, and his actions often reveal a deep-seated protective instinct toward his sister. In one instance, after a classmate cruelly rejects Akane's confession of love by calling her stupid-looking, Aoi defends her honor, countering that the classmate is the foolish one. This act of defense, however, paradoxically infuriates Akane, who does not appreciate his interference in her romantic life, further complicating their relationship.

Beyond his academic prowess and cool exterior, Aoi demonstrates a thoughtful and caring side that points toward his personal aspirations. He reminds Akane of a promise she made to him when they were children, and he confesses his own dream to become a veterinarian. This ambition reveals a gentle and nurturing quality that lies beneath his reserved surface. Furthermore, he expresses a firm desire to protect his sister, solidifying his role as her guardian despite her protests. Aoi does not appear to have many notable abilities outside of his scholastic excellence, but his quiet determination and sense of responsibility are key strengths that guide his actions throughout the narrative.