Musubu Takatsuki is the younger sister of Tazuna Takatsuki, the protagonist of the series. She died from an illness before the events of the story. Her birthday is September 20. She is depicted as a teenage girl with short black hair and blue eyes. While little is directly shown about her personality, Tazuna remembers her with deep affection, and her dying wish was for him to never let go of her hand. This wish becomes a central emotional anchor for Tazuna throughout the series. In the narrative, Musubu appears only in flashbacks and memories. Her death is a key part of the backstory: it is implied that her organ donation contributed to the survival of another character. More importantly, when Tazuna first meets Koyori Akutagawa, a bedridden girl in a university research facility, he is strongly reminded of Musubu. This resemblance prompts him to take Koyori’s hand, awakening his power as a Hand Shaker and binding their lives together. Musubu’s memory thus serves as a motivating force for Tazuna’s actions and his determination to protect Koyori. Her relationship with Tazuna is the most significant; she is a source of both grief and inspiration. As the story progresses, Tazuna gradually learns to see Koyori as her own person rather than a replacement for his sister. Musubu herself does not undergo any development, as she is deceased, but her legacy influences Tazuna’s emotional growth and his approach to his partnership with Koyori. She possesses no special abilities; she was an ordinary girl.