Miyuki Sakaki, a classmate of protagonist Kotomi, sports long black hair usually in a braid, brown eyes, a slim physique, and pale skin. She typically wears her school uniform adorned with a ribbon hair accessory and a string ribbon tie. Miyuki maintains a consistently stoic, expressionless demeanor, rarely showing her emotions outwardly. This reserve stems from a profound personal tragedy: her younger sister's murder one year before the story's events. This loss fuels a deep distrust of men and an unwavering focus on identifying and confronting her sister's killer.

Her background includes the discovery of a deeply personal, incriminating note in her bedroom during summer vacation. This note forces her compliance, compelling Miyuki and classmates including Kotomi to gather at the old school building. While the note's exact contents remain unspecified, its invasive nature drives her actions. At this gathering, Miyuki emerges as a headstrong presence among classmates who appear to be victims of similar blackmail attempts.

Despite her outwardly cautious and serious personality, Miyuki possesses a hidden inner passion. Her relentless pursuit of justice for her sister reveals this underlying intensity. The investigation into her sister's death and the circumstances forcing her to the old school building form the core drivers of her narrative actions.