Fuyutsuki, whose first name is Mio, is a young woman of average height with wavy, shoulder-length brown hair and matching brown eyes. Her typical expression is neutral and calm, though she does occasionally display moments of smiling or bashfulness. At work, she dresses in standard office attire such as blouses, skirts, and high heels, while her casual clothing outside the office is varied and stylish. She is a completely normal human being with no supernatural lineage, in contrast to several of her colleagues.

Her defining characteristic is her calm, quiet, and reserved demeanor, which has led people in the past to perceive her as stoic, cold, or unemotional. This perception negatively impacted her self-worth during her high school years, leading her to believe she was boring. Despite this external coolness, she is deeply kind, thoughtful, and helpful. She is a very observant and dependable person who demonstrates genuine care for others through her actions rather than grand gestures. Fuyutsuki is also pragmatic, matter-of-fact, and at times oblivious to romantic cues and social expectations. Her relationship with Himuro draws out a more expressive side of her personality, revealing hidden depths of warmth and even protectiveness.

Fuyutsuki begins working at the same company as Himuro, and their first meeting occurs before they discover they are colleagues, when she finds him stuck in place due to his own ice powers. Her role in the story is that of the supportive and grounding presence for Himuro. She is never fazed by his supernatural ice powers, which activate when he experiences strong emotions. Instead, she calmly observes his struggles and proactively finds creative solutions to help him navigate a world not designed for his abilities, such as bringing in straw to insulate flowers he likes or gifting him her cat's shed whiskers for good luck. Her primary motivation stems from her own past experiences of being misunderstood. Understanding how it feels to be left out or judged for her reserved nature, she works to ensure Himuro does not feel the same way, striving to help him experience things that would otherwise be impossible for him.

Her most significant relationship is with her coworker, Yukiya Himuro. She has been interested in him since their first encounter, and they naturally grow closer while working in the same department. Although she is initially inexperienced with romance and slow to recognize her own feelings, her affection for him is shown through quiet, thoughtful gestures and consistent support. As the story progresses, she becomes more aware of her emotions, and by chapter 54 of the manga, she and Himuro are officially in a relationship. Her protectiveness over him is made evident when she asserts that he is hers in front of their colleagues. She also develops a close friendship with her coworker Komori and a friendly relationship with Saejima. She is well-regarded by Yukimin, Himuro's younger sister, who has a girl-crush on her and visits her apartment. Fuyutsuki also owns a cat named Nyamero.

Over the course of the series, Fuyutsuki experiences notable personal development. Her interactions with Himuro help her become more expressive and signal a departure from the stiff, cold image that plagued her in the past. While she remains reserved, she begins to open up emotionally and becomes more confident in showing her affection in her own subtle way. Her main ability is her lack of supernatural powers. Her strength lies in her emotional maturity, her unflappable composure in the face of Himuro's magical mishaps, and her profound empathy, which allows her to build a deep connection with him based on acceptance and mutual respect.