Koyuki Ayase attends Sakuragaoka High School, starting as a third-year before moving on to college. He sports a white-to-pink gradient hairstyle and blue eyes. Previously, during middle school and early high school, he had long, shoulder-length hair, wore red-framed prescription glasses, and favored long-sleeved clothing to hide his figure, a presentation perceived as feminine that resulted in bullying. In his third high school year, he transformed his appearance, cutting his hair short, switching to contact lenses, and adopting standard school uniforms, including short sleeves in summer.

His personality, shaped by a poor self-image stemming from bullying over his effeminate looks and name, led to low self-confidence, social withdrawal, and avoidance of physical activities. He disliked being called "cute" but found refuge in reading manga, forming a connection with Natsuki Enomoto through this shared interest. Post-transformation, his confidence increased though he remained somewhat reserved. He developed a capacity for assertiveness, exhibiting passive-aggressive tendencies and possessiveness during confrontations with Yu Setoguchi. Internally, he struggled to express emotions directly, often using hypotheticals to convey feelings. Newfound popularity brought unwanted attention, causing discomfort with nicknames and difficulty forming genuine friendships, fueling envy towards Yu's social ease.

Koyuki's romantic focus centered on Natsuki Enomoto, whom he loved since middle school because she treated him kindly and used the respectful "Koyuki-kun" instead of the feminine "Yuki-chan." Their bond deepened through manga discussions. Hoping to impress her, he changed his appearance in his final high school year. He took her on a date and attempted a confession, escalating into a confrontation with Yu, who also loved Natsuki. Witnessing Natsuki's distress during their argument and later her successful confession to Yu led Koyuki to accept his unrequited feelings. He came to regard Natsuki as a cherished first love, supporting her happiness from a distance.

Hina Setoguchi developed feelings for Koyuki after an accidental encounter in middle school where he saw her underwear, leading to a chase and his apology. She pursued him into high school, joining the Gardening Club where he tended plants. After his rejection by Natsuki, Hina confessed her love, but Koyuki misinterpreted this as pity and dismissed her. Their relationship stayed platonic; he admired Hina's confidence and strength, believing she deserved a partner who truly loved her.

His friendship with classmate Sota Mochizuki developed neutrally until Sota accidentally used Koyuki's nickname "Yukki." Learning of Koyuki's loneliness, Sota sought genuine friendship. Koyuki later advised Sota to promptly confess his feelings for Akari Hayasaka, emphasizing that trying and failing was better than inaction.

Koyuki saw Yu Setoguchi as a romantic rival due to Natsuki's affection for him. Their interactions ranged from minimal contact to confrontational, particularly after Koyuki's date with Natsuki, where he criticized Yu for making her cry and demanded a "fair competition." He envied Yu's close friendships, contrasting with his own superficial popularity.

Kotaro Enomoto, Natsuki's younger brother, initially resented Koyuki for Hina's affection. After Koyuki rejected Hina, Kotaro confronted him. Koyuki redirected Kotaro to pursue Hina himself, acknowledging Kotaro's feelings and advising against future regrets.

Post-high school, Koyuki became a florist, maintaining his gardening interest. In college, he met Ryou Ogino, an upperclassman, through his florist job and shared anime communities, leading to his first reciprocated romantic relationship. His name, meaning "love" (恋) and "snow" (雪), contributed to childhood teasing due to its feminine associations.