Koyuki mixes drinks at Shin-Yokohama Highball, the bar run by the Vampire Hunters' Guild. She is the daughter of Gousetsu, the guild's manager and bar owner. Beyond bartending, she also trains as an intern vampire hunter.

She sports black hair in a short bob and brown eyes. Her standard work outfit is a professional bartender's uniform: a white dress shirt, black vest, red bowtie, black pants, and black shoes.

Koyuki is notoriously incapable of cooking, resulting in a strict ban from entering kitchen areas. However, she displays exceptional skill at the game yakyuken, consistently defeating even the specialist known as Yakyuken Lover. In her manga appearances, her dialogue uniquely lacks conventional speech bubbles.