Yui Aoi stands as a central figure in her sixth-grade class narrative, sharing a close three-girl friendship with Hanabi Ayase and Marin Ogura. Within this group, she experiences the earliest noticeable physical and emotional development, yet harbors deep insecurities rooted in her mother's absence. This lack of maternal guidance drives her to frequently seek advice from Marin, who often imparts wisdom gleaned from her more experienced older sister. Yui initially maintains a composed, model-student demeanor and holds distinct disapproval for Kazuma Hiyama, viewing his delinquent tendencies and habitual teasing as marks of immaturity and disruption.

Her perception of Kazuma shifts dramatically after two linked events. Faced with a malfunctioning shower at home, she starts using the bath at Kazuma's residence. During these visits, she loses a key left by her late mother—a possession of immense sentimental value. Kazuma discovers this key and undertakes considerable effort to return it to her, revealing an unexpected considerate side. Concurrently, he steps in to protect her when another classmate spreads malicious gossip behind her back. His defense of her reputation, combined with his determined restoration of her cherished memento, sparks a shift in her feelings. She gradually transitions from dislike to affection, recognizing a softer dimension beneath his outwardly brash personality.

This evolving connection culminates in mutual confessions, establishing Yui and Kazuma as a romantic couple. They become recognized as the second pairing within their classroom, following Hanabi and Yuuto Takao. Witnessing her best friend Hanabi navigate romance contributes to Yui's own burgeoning desire for love. Her journey with Kazuma thereafter involves navigating typical sixth-grade social dynamics and the physiological changes of early adolescence. Her foundational character arc centers on overcoming initial prejudices, embracing vulnerability, and discovering mutual affection through Kazuma's consistent, though initially obscured, kindness and protective nature.

Physically, Yui is characterized by light brown hair and brown or olive-colored eyes, fitting her pre-teen age. Her background remains anchored in growing up without maternal support, a void subtly influencing her interactions and amplifying the significance of Kazuma's thoughtful gesture involving her mother's key. Her development illustrates a transition from judgment based on superficial behavior to an appreciation for deeper character traits, framed within the complexities of first love during a pivotal life stage.