Yui Aoi is a primary protagonist alongside Hanabi Ayase, forming a close-knit trio of friends with Hanabi and Marin Ogura. She displays earlier physical and emotional maturity than her peers. Lingering insecurity from her mother's absence leads her to frequently seek Marin's guidance on personal challenges.

She initially detested Kazuma Hiyama for his delinquent behavior and teasing, resenting him further when classmates influenced by him targeted her with pranks. Her feelings transformed after Kazuma defended her against malicious gossip. A subsequent incident—her shower malfunction necessitating baths at Kazuma's home—culminated in her losing a cherished key belonging to her late mother. Kazuma's dedicated effort to recover the key deeply moved her, sparking romantic feelings. She confessed, and they became their class's second established couple.

Their relationship faced strain when Inaba Mikami, a popular cram school peer, pursued her upon learning she had a boyfriend. Kazuma's jealousy over her interactions with Inaba created tension, thrusting Yui into a complex dynamic where she navigated both suitors' attentions, testing her commitment and emotions. Despite projecting a model student image, she continuously grappled with personal insecurities and the intricacies of her first romance throughout these experiences.