Riku Momosaki, once a junior high teammate of Minato Kiyomizu, now commands attention as the ace of Shogakukan High’s water polo team. Initially a swimmer, his pivot to water polo inadvertently shifted the trajectory of his younger brother, Shūgo Amihama, who had aspired to surpass him in the pool before distancing himself from the sport.

Riku’s sharp features—sleek midnight hair swept back, angled brows, and piercing blue eyes—mirror his intense demeanor. His uniform, casually unbuttoned at the top, hints at a duality of discipline and ease. Emotionally charged, he reacts with raw fervor upon learning of Minato’s memory loss, lunging onto his hospital bed in a desperate bid to jolt his friend’s recall, only restrained by their team advisor. Yet beneath this impulsivity lies empathy, as he supports Minato despite the strain of their fractured connection.

Blunt yet earnest, his sincerity invites frequent teasing from peers, often leaving him flushed. He tempers his rashness with unexpected composure in interactions involving Shūgo, revealing a guarded maturity in brotherhood. A member of Shogakukan’s “Elite Four” alongside Keita Kakihana and Kōki Toguchi, Riku stands as a formidable rival in water polo’s competitive sphere.

His name, “Riku” (陸) meaning “land,” contrasts with the surname Momosaki, derived from “hundred” (百) and “cape” (崎), anchoring his identity in both steadfastness and uncharted edges.