Aya Tachibana is the protagonist of the series, a sixth-grade girl whose life changes dramatically when she becomes a member of the Detective Team KZ. Before this encounter, she had no friends, largely due to her cautious nature and a deep-seated fear of standing out or drawing attention to herself, which often leads to embarrassment. This social anxiety is compounded by a personal complex she harbors about being a neglected middle child within her family. Academically, her strongest subject is Language Arts, which becomes a key asset to the team, while her weakest is Mathematics. She is often referred to by her nickname, Aaya.

As the team’s Language expert, Aya is responsible for recording the details of their investigations in her notebook. She sees herself as the sensible, normal member of the group, a stark contrast to the four eccentric boys she works with. Her role is not just clerical; her observations and way with words are crucial to their process. Initially, she is overwhelmed and mortified by the boys’ antics, particularly their willingness to be the center of attention, but she gradually finds her place among them. The team is comprised of the arrogant and attention-grabbing leader Kazuomi Wakatake, the mysterious social expert Takakazu Kuroki, the stoic math prodigy Kazunori Uesugi, and the kind-hearted science and social studies whiz Kazuhiko Kozuka. Her relationship with Wakatake is particularly contentious; his brash and insensitive nature causes friction, yet there are subtle hints of a deeper, more jealous dynamic between them as the series progresses.

Aya’s development centers on overcoming her social insecurities and learning how to connect with others. She starts as a lonely girl who actively tries not to stand out, but her involvement in the detective team forces her to confront her self-inflicted social failures. She learns to navigate friendships and even experiences complex feelings like jealousy, both when she learns of Wakatake’s past romantic interests and when the team notices her own close bond with another boy, Kakeru Sunahara. Her journey is about discovering that having friends and being part of a group, despite the bickering and challenges, is a positive change. Her notable ability is her proficiency with language, which allows her to articulate observations clearly and maintain the case notes that document the team’s activities. She proves to be a more complicated and engaging character than typically expected in a children's mystery series, showing neediness and a temper while remaining the relatable heart of the team.