Yūki Kusakabe presents a calm, refined, and subtly secretive demeanor with a romantic nature. She possesses a slender, tall frame, long black hair, deep blue eyes, and is typically seen in her school uniform: a miniskirt, loafers, and ribbon tie. Originally named Takashiro Yūki, she shared her childhood with Takaaki Kouno. After her parents' divorce forced a move and a loss of both friends and surname, Takaaki offered her his own name, suggesting she could become "Kouno Yūki" if unhappy with her new one—a gesture later interpreted by the robot maid Harumi as a marriage proposal, casting Yūki as a romantic rival. Years later, she returned to her hometown bearing the surname Kusakabe.

Reserved and nostalgic, Yūki often haunts the night, carrying her "Trash Basket Notebook" for stargazing and personal reflections. Her behavior includes daydreaming and a fond attachment to the memory of Takaaki's childhood name offer. She embodies the Yamato Nadeshiko archetype, engaging in teasing and maintaining a mysterious presence, rarely appearing during school daylight hours.

Her story involves reconnecting with Takaaki after their long separation. Harumi perceives Yūki as a rival due to the implied significance of Takaaki's name-giving, fueling a comedic conflict rooted in unresolved childhood bonds and misinterpreted promises. Yūki's measured reactions and persistent calmness contrast with Harumi's assertiveness.

Physically, she stands 157 cm tall with measurements of 84-54-80 cm (bust-waist-hips). Her birthday is November 3.