Grandmother, whose full name is Fuzuki Koyama, is the strict and formidable maternal guardian of the protagonist, Mitsuki. She is an older woman who lives in a traditional Japanese home with Mitsuki and a maid, and her demeanor is initially defined by her stern, unbending nature. Her primary role in the story is as an obstacle to Mitsuki's dream of becoming a singer, as she has a deep-seated and vehement hatred for music. This hatred manifests in rigid household rules where Mitsuki is forbidden from humming, singing, or even listening to music, with punishments often following any transgression. From her perspective, her harshness is an expression of love and protection. She believes that music is a destructive force that has stolen everyone she has ever loved, and she is determined to prevent it from taking her granddaughter as well.

This profound aversion to music is rooted in Fuzuki's painful past. She was deeply scarred when her daughter, Hazuki, fell in love with a musician, Aoi Koga, and ran away from home to be with him, a decision that ultimately led to her untimely death. Adding to this trauma, Fuzuki herself had once loved playing the piano, but her own fiancé abandoned her, which she also came to blame on her passion for music. These cumulative losses caused her to shut herself off emotionally, hardening her heart against the very art form she once cherished. Her strict control over Mitsuki's life, including insisting on a surgery that would save her life but rob her of her voice, is a misguided attempt to ensure her granddaughter's safety and longevity, even at the cost of her happiness.

Despite her cold exterior, Fuzuki's relationship with Mitsuki is central to her character development. She sees Mitsuki not only as her grandchild but as a living reminder of the daughter she lost, which makes her simultaneously protective and pained. Throughout the series, she remains unaware for a long time that her frail, 12-year-old granddaughter is secretly living a double life as Full Moon, a healthy 16-year-old pop star. Her key relationships outside the home include her interactions with Keiichi Wakaouji, the family doctor who was also a friend of Mitsuki's father. He often serves as a mediator, trying to help Fuzuki understand Mitsuki's feelings.

Fuzuki undergoes significant character development as the story progresses. A pivotal moment occurs when she sees Mitsuki in a moment of extreme despair, which cracks her rigid exterior and allows her to finally express the love she has been holding back. This event forces her to confront the fact that her overbearing protectiveness has caused her granddaughter immense pain. In a powerful shift from antagonist to ally, she eventually comes to respect Mitsuki's dreams and free will. She learns to let go of her hatred for music, even expressing a desire to hear Mitsuki sing, and works to support her career while also ensuring her health is monitored. While she does not possess supernatural abilities like the shinigami, her notable power lies in her iron will and authority over her household. In her past, she was also a pianist, though she abandoned this talent after her fiancé left her. By the end of the series, Fuzuki transforms from the primary figure holding Mitsuki back into one of her most steadfast supporters, finally reconciling her love for her family with the memory of the music that once tore them apart.