Shugo Amihama, a Yamanami High School student, once channeled his competitive drive into swimming during junior high, fueled by the desire to outshine his older brother, Riku Momosaki. When Riku shifted to water polo, Shugo abandoned the sport, his purpose evaporating without his rival. Their fractured family history—divorced parents, Riku living with their mother while Shugo stayed with their father, an adopted son-in-law bearing a distinct surname—deepened their rivalry, intertwining competition with a quest for recognition. Riku himself had taught Shugo to swim, embedding a rivalry that lingered even as their lives diverged.

Recruited by water polo captain Jō Jōjima, Shugo resisted until challenging Jō to a race: victory would secure his spot on the team. Though he won, his true motivation emerged upon learning Riku now played for Shogakukan High, reigniting his resolve to surpass his brother in this new arena.

Shugo’s stoic discipline and expressionless demeanor earned him nicknames like “The Mask” and “Noh mask,” masking a sharp analytical mind that adapted strategies mid-match. Gradually, he forged mutual respect with teammates such as Minato Kiyomizu, acknowledging their shared dedication even as his focus remained fixed on defeating Riku.

Standing 6’1” with a lean build, black hair, and piercing blue eyes, his athleticism is mirrored in his precise appearance—neatly pressed uniforms, coordinated blue-and-white team gear. Silent determination anchors his role as a pivotal player, balancing personal ambition with team cohesion.

Notably, Shugo claimed third in junior high’s 100-meter freestyle, while his name—combining “excellent” and “seaside/net”—echoes both his prowess and the familial ties that shape his journey.