Fuzuki Kouyama is a supporting character known as the maternal grandmother of the protagonist, Mitsuki Kouyama. In her youth, Fuzuki was best friends with a wealthy girl named Moe Rikyo. Despite dealing with her own illness and being pressured into an arranged marriage with a businessman, Fuzuki prioritized her friend's happiness. When they both met the violinist Seijurou Koga, Moe developed feelings for him, and Fuzuki selflessly stepped aside, choosing not to compete with her friend. The three became a close trio until a series of tragic misunderstandings occurred. Moe was told Fuzuki was too ill to see her and was being forced into a marriage with Fuzuki’s former fiancé. In a final act of sacrifice, Moe witnessed Fuzuki sharing a last kiss with Seijurou, a gesture meant to step aside for Moe, but it led Moe to take her own life. This event devastated Fuzuki and led to her definitive separation from Seijurou.

Years later, a still embittered Fuzuki strongly disapproved of her own daughter, Hazuki, pursuing a relationship with Aoi Koga, Seijurou’s son and a musician. When Hazuki eloped with Aoi while pregnant, both later died, leaving Fuzuki with profound regret for not having reconciled with her daughter. She took in her orphaned granddaughter, Mitsuki, but remained cold and strict, forbidding the girl from singing. Her hatred of music is rooted in the belief that it has taken away everyone she has ever loved, and she fears it will take Mitsuki as well. Despite this harsh exterior, her actions are driven by a deep-seated desire to protect her granddaughter from the pain she herself has endured. When Mitsuki runs away to pursue her dream of becoming a singer, Fuzuki worries greatly and gradually comes to understand that she cannot suppress the girl's passion for music.

Fuzuki serves as a primary obstacle and a source of generational trauma in the story. Her relationship with Mitsuki is central to the narrative, representing a conflict between protection through repression and the need to follow one's dreams. Her key relationships are framed by tragedy: her lost friendship with Moe, her complicated romance with Seijurou Koga, and the devastating rift with her daughter Hazuki. Over the course of the series, Fuzuki experiences significant development. She begins to soften from her bitterly cold demeanor as she witnesses her granddaughter's unwavering determination. By the end of the story, she is able to reconcile with her past, including reuniting with Seijurou on friendly terms during one of Mitsuki's concerts, indicating that she has finally found peace and can embrace music once more. Regarding notable abilities, Fuzuki has no supernatural powers or special musical talents; her significance is purely narrative, grounded in her emotional journey and her role as a grieving mother and grandmother.