Ruko Ayase functions as Yuuto Ayase's older sister. Their parents are perpetually absent due to work, leaving Ruko nominally in charge of the household – a duty she utterly neglects. While she maintains a professional and serious demeanor at her job, her behavior undergoes a drastic reversal at home. There, she abandons all responsibility for heavy drinking and partying, frequently alongside her equally irresponsible friend Yukari Kamishiro. These gatherings often leave their living space resembling a trash-strewn chaos, with Ruko typically found lounging scantily clad in the aftermath. She possesses exceptional alcohol tolerance, described as reaching almost inhuman levels. Ruko exhibits complete domestic incompetence, relying wholly on Yuuto for essential tasks like cooking, cleaning, and general household upkeep. She asserts dominance over him, barking sharp, impatient commands and readily shouting her irritation if he delays meal preparation. Physically, she has black hair, red eyes, stands 167 cm tall, and has measurements of 82-58-84. Her brash, confrontational attitude and style of dress, particularly her quickness to threaten or brandish her katana against perceived disrespect or annoyance, draw comparisons to Revy from *Black Lagoon*. She briefly appears in Episode 4 of the main series, encountered by Yuuto, Haruka, Mika, and Nanami during their surveillance of maid Hazuki Sakurazaka. She also appears in the OVA *Nogizaka Haruka no Himitsu: Finale*, maintaining her established role within the Ayase household.