Yūki Kusakabe appears as a mysterious girl, often seen at night within the school setting while carrying a notebook known as the Trash Basket Notebook. She is the childhood friend of protagonist Takaaki Kouno, fondly recalling a past memory where he suggested she could use his last name if she disliked her own. An Episode 13 flashback shows her as a young girl crying over an impending move.

Physically slim and tall at 157 cm, Yūki possesses long black hair and deep blue eyes. Her personality is generally calm and caring, yet maintains a reserved demeanor, interacting quietly with others during nocturnal hours.

In the spin-off role-playing game *To Heart 2: Dungeon Travelers*, Yūki is a recruitable party member of the "maid" occupation. Transported to a fantasy world following a science experiment, her goal is to help defeat the final boss to return home. She participates in turn-based battles, utilizing skill-based actions unique to her class progression that contribute to party cooperative mechanics. She can advance to "Bard" or "Dance" at level 15, and later to "Diva," "High School Girl," or "Idol Master" at level 30.

Her depiction across media maintains consistency in emphasizing her mysterious presence and connection to Takaaki.