Ryūsei Midorigaoka, a once-promising tennis prodigy, abandons his athletic career following a debilitating wrist injury that reshapes his identity, fostering a sharp-tongued, prideful persona. Despite his academic excellence and revered status as an honor student, he harbors unwavering reverence for his enigmatic mentor, Soshi Momoya, emulating the teacher’s intellectual rigor.

A childhood bond with Junpei Shiroyama fractures when Junpei opts for the gardening club over tennis, seeding years of unresolved rivalry. During a dimensional clash with the Sphinx Dictis, Ryūsei is briefly erased from existence after the group falters in solving a riddle. Rescued but irrevocably altered, he loses his defining pride, adopting a disarming, socially inept demeanor.

Stripped of arrogance, he candidly acknowledges his injury’s permanence and passionately campaigns for the tennis team he can no longer join. Junpei later exposes the injury’s severity during a subsequent battle, contextualizing Ryūsei’s abrupt retirement. Following Dictis’s defeat, Ryūsei’s restored pride reignites his former haughtiness.

Guided by Tsuyoshi Kikawada’s intervention, he mends ties with Junpei. A non-canon dream sequence later portrays him collaborating seamlessly with peers to unravel puzzles—a fleeting glimpse of uncharacteristic unity. Driven by loyalty to Momoya, he probes the teacher’s sudden disappearance, uncovering links to the deceased Shiro Oide and the Sphinxes’ inter-dimensional machinations.