Daisuke Aoki is a bespectacled, good-natured 23-year-old homeroom teacher starting at Futatsubashi Elementary School's Class 3-1, later shifting to 4-1. Recently graduated from university, his uneventful childhood featured above-average academic and athletic skills, including substituting for his university track and field club. During those years, he developed unrequited feelings for a female peer who married his senior before he confessed. He held no resentment, believing her happiness was paramount.

Appointed mid-trimester as Nakamura's replacement, his inexperience became an immediate hurdle. He grappled with controlling his unruly class and overcoming predecessor-induced mistrust. His idealism faced daily tests, particularly from eight-year-old student Rin Kokonoe, who aggressively pursued him with sexual innuendo, threats of false misconduct accusations, and blackmail. His lack of romantic experience heightened vulnerability to her advances and to virginity taunts from student Kuro Kagami.

Enthusiastic and emotionally invested, he employed hands-on teaching that clashed with strict colleague Sae Shirai’s traditional methods. His tendency to overanalyze, obliviousness to romantic interest, and poor social cues complicated interactions, especially with Shirai, who disliked him. Senior teacher Kyōko Hōin consistently provided him practical advice and moral support.

His relationship with Rin evolved dramatically. Initially troubled by her behavior, he softened upon learning she was an orphan. He sought to understand her motivations, and she occasionally used her influence to aid his classroom discipline. A late-year confrontation forced him to acknowledge a strong attraction to her. Confused, he interpreted these feelings as paternal love after Mimi Usa’s comment—though he frequently doubted this—spurring protectiveness toward Rin and distrust of her guardian, Reiji Kokonoe.

When conflicts arose between Rin and Reiji, he moved into their home to monitor the situation. He later moved out at Shirai’s insistence after she witnessed him leaving at night. Ultimately, he admitted his romantic feelings for Rin but chose inaction, prioritizing her well-being and supporting her happiness until maturity. He accepted she might outgrow her feelings. They reunited romantically after Rin turned 16.

He has a younger sister, Chika. His surname "Aoki" means "blue, green tree," and "Daisuke" means "big help."