Ruko Ayase, older sister to Yūto Ayase, assumes de facto household leadership with her brother due to their parents' frequent work absences. Despite this role, her domestic capabilities are minimal, particularly after alcohol consumption, forcing Yūto to handle all chores. Professionally disciplined, she works as a private secretary to a company president.

Ruko typically displays a rough, brutish demeanor but exhibits uncharacteristic warmth and approval upon meeting Haruka Nogizaka, Yūto's classmate. A dedicated weapon enthusiast, she owns numerous weapon magazines and a personal Japanese blade named "Ruri Dokuro" (Lapis Lazuli Skull). Her combat proficiency is exceptional, holding a 2nd-degree karate black belt and demonstrating top-class battle skills, evidenced when she infiltrates the Tennōji family's high-security estate alongside Yukari Kamishiro during Yūto's temporary butler employment.

She maintains a close friendship with Yukari Kamishiro, Yūto's teacher, frequently drinking heavily together at the Ayase residence. These sessions create domestic disarray, necessitating Yūto's cleanup and meal preparation. No significant character evolution or additional background details beyond these traits and incidents occur across the series' seasons, films, or OVAs.