Shirou Mizunuma fulfills his role as Konan Academy's student council president with evident leadership and organizational aptitude. He maintains a close friendship with Shun Kazama, partnering on projects like the school newspaper and championing the preservation of the Latin Quarter clubhouse—a vital student activity center. His efforts to save the Latin Quarter involve coordinating student demonstrations, displaying protest banners, leading renovations, and traveling to Tokyo with Shun and Umi Matsuzaki to personally persuade school board chairman Tokumaru. Their appeal succeeds, resulting in the building's preservation.

Academically, Shirou excels in physics, expressing visible dissatisfaction when denied a perfect exam score. He publishes his physics test in the school newspaper and references his older sister's astrophysics career with pride. His personality blends assertiveness and composure, often tempering Shun's impulsiveness. He demonstrates respectfulness, such as when politely requesting Umi's help with the newspaper due to staffing shortages. Habitually checking his wristwatch underscores his attention to detail, while his perceptiveness allows him to notice and tactfully address growing tension between Shun and Umi.

His interactions with Sora Matsuzaki evolve from her initial admiration for Shun into a mutual connection. He invites her to a student assembly, and their increasing interactions culminate in Sora extending a farewell party invitation. During a confrontation at the Latin Quarter, Shirou defuses tension by initiating the group's anthem, uniting the students through song.