Tōma Aizen serves as captain of Soukai Academy's understaffed Gunpla Club, driven by the need to support his ill mother and younger siblings. His goal of becoming a professional Gunpla fighter aims to secure a better life for his family. At Soukai, he exhibits a cool-headed and observant personality, prioritizing his teammates' problems and approaching battles with focus. His rivalry with protagonist Ryusei Fudou fuels mutual growth through competition and shared training.

Securing a professional contract necessitates transferring to Gokusode University's affiliated high school as a sponsorship condition. This decision sparks a significant rift with his former Soukai Academy teammates. Following his transfer, Tōma's personality shifts markedly. He transforms from a calm, supportive figure into a more competitive, cold, and harsh fighter. He explicitly designates the protagonist as his rival, becoming singularly focused on surpassing them. This manifests in aggressive battle tactics, despite his outwardly composed demeanor. His combat style thus presents a distinct contrast: a cool exterior masking bold and aggressive Gunpla strategies.