Ruko Ayase lives with her younger brother Yūto in their family home, acting as a primary authority figure due to their parents' frequent work absences. She works professionally as a private secretary for a company president. Ruko displays a rough, brusque demeanor in speech and attitude. Despite her work dedication, she engages in heavy drinking at home, rendering her incapable of household chores and leaving Yūto to manage all domestic responsibilities.

She maintains a close friendship with Yukari Kamishiro, Yūto’s homeroom teacher, whose visits often escalate into drinking sessions that disrupt household management. Ruko exhibits immediate and strong approval upon meeting Haruka Nogizaka, Yūto’s classmate and love interest—an unusual reaction given her typical disposition.

Ruko possesses advanced combat skills, holding a 2nd-degree black belt in karate, and is an avid weapon enthusiast with an extensive collection of weapon-related magazines. Her personal favorite is a Japanese blade named "Ruri Dokuro" (Lapis Lazuli Skull). She demonstrates this proficiency when infiltrating the high-security Tennōji estate alongside Yukari Kamishiro to locate Yūto during his temporary butler employment there, after he neglected to prepare their meals.