Kaoruko Serigaya, a third-year at Kounan High School and key badminton club member, possesses long pink hair initially in twin tails and pink eyes, later cut shoulder-length. She exudes confidence and boastfulness with ojou-sama affectation, comparing herself to a butterfly others naturally follow, and initially claims no rivals in prefectural tournaments. Despite this arrogance, she shows empathy, particularly toward Ayano Hanesaki, helping her recognize a desire for kindness, and harbors feelings for Kentaro Tachibana, whom she considers cool.

Their rivalry traces to middle school, where Kaoruko intentionally exposed Ayano to illness before a critical match, contributing to Ayano's loss and her mother's abandonment. This strained their relationship, though Kaoruko later gave Ayano a handkerchief to dry tears—repurposed by Ayano as a hair tie. Kaoruko consistently loses to Ayano in matches, including a 0–2 Inter High Preliminaries quarter-finals defeat.

As a player, she employs a control-oriented style using data analysis and technical precision, backed by mental resilience, diligence, and aversion to defeat, enabling mid-match adjustments. Her record includes winning a middle school prefectural tournament and competing nationally. Favored in Inter High qualifiers, she advanced only in doubles with partner Miki after her singles loss to Ayano, utilizing psychological tactics against opponents.

Beneath her haughty exterior, Kaoruko views her dynamic with Ayano as a natural bond between driven individuals, asserting they are "meant to love each other." Teammate Miki acknowledges Kaoruko's hidden kindness, indicating her arrogance masks complexity. Her pragmatic competitiveness and emotional evolution regarding Ayano and Kentaro underscore her beyond a mere rival.