Yūki Sasamori is a first-year student at Kunpō High School and a classmate of protagonist Kouta Oyamada. She shares a close friendship with Kiriko Takana, and their frequent activities often fuel school gossip. Yūki has brown hair and blue eyes.

She possesses a notably wild and quick-witted personality, frequently displaying mischievous behavior. Yūki actively spreads rumors concerning Kouta, especially those detailing his interactions with the fox spirit Chizuru Minamoto. She is the originator of Kouta's nickname "Great Erotic King" or "Great Eros King," claiming Chizuru's overt romantic advances stem from Kouta's perceived irresistible aura.

Her conduct often involves describing or exaggerating intimate encounters between Kouta and Chizuru, seemingly deriving enjoyment from these accounts. This contrasts with the more disciplined attitude of class representative Akane Asahina, who strives to maintain order. Yūki further teases Akane, mocking her for having a "dirty mind," which complicates Akane's efforts to enforce decorum.

Yūki consistently aligns with the student body's stance on various matters during her appearances. Her primary role involves instigating or amplifying school gossip, particularly regarding supernatural relationships among the students.