Yu Ishigami serves as treasurer on the Shuchi'in Academy student council. His family owns a small toy manufacturing business, and his mother works as a florist. Admitted to the prestigious academy largely through his father's efforts, Ishigami was not inherently academically driven.

During middle school, a strong sense of justice led him to intervene upon discovering classmate Kyoko Ootomo's boyfriend, Ko Ogino, cheating on her and distributing explicit images. Ishigami confronted Ogino, physically assaulting him after Ogino tried bribing him with access to Ootomo. Ogino then publicly framed Ishigami as acting from jealous obsession, resulting in severe ostracization, a long-term suspension, and deep damage to Ishigami's self-esteem. His refusal to write a demanded apology letter worsened the situation. The isolation eroded his academic performance and social confidence.

Physically slim with chin-length black hair deliberately covering his left eye as a barrier reflecting his fear of others, Ishigami lost his former athletic physique from middle school due to inactivity. His personality is initially gloomy, cynical, and reclusive, expressing disdain for social norms and popular students ("normies"). He avoids social interactions, preferring video games, manga, and anime music, though he conceals these interests to avoid the otaku label. Despite this, he possesses keen observational and analytical skills, frequently noticing hidden insecurities in others—a trait that sometimes causes unintentional social friction.

His trajectory shifts when student council president Miyuki Shirogane investigates the middle school incident and recruits him for his data-processing abilities, forcing Ishigami to attend school regularly. Key developments include joining the cheerleading club to fulfill requirements, where he interacts with third-year student Tsubame Koyasu. He develops feelings for her and inadvertently confesses during the cultural festival by gifting a heart-shaped item, unaware of its traditional symbolism. Tsubame initially considers gentle rejection but later dates him seriously before ending the relationship due to age differences and platonic feelings. This rejection initially depresses him but catalyzes self-improvement efforts.

Academically, Ishigami rises from 197th to 8th in class rankings through dedicated effort. Socially, participation in events like the sports festival—where he nearly wins a relay race despite resurfacing past trauma—helps him reconcile with his history. Peer support during these moments fosters his gradual acceptance of others and reduces isolation.

Relationships define his growth. His bond with Shirogane is foundational, built on mutual trust. With Kaguya Shinomiya, he evolves from fear to earning her respect through sincerity. His dynamic with Miko Iino, known since middle school, is adversarial yet underpinned by mutual, anonymous support: he once sent her encouraging flowers during her bullying, while she mitigated his punishments. Their friction continues in high school, but after Iino breaks her arm saving him, he becomes attentive to her. Post-Tsubame’s rejection, Ishigami recognizes developing romantic feelings for Iino. He also forms a friendship with Maki Shijo over shared romantic frustrations, helping her move past her crush.

Post-graduation, Ishigami remains at Shuchi'in as vice president under Iino’s presidency, placing them in a dynamic mirroring Shirogane and Shinomiya’s initial relationship.