Koyori Ishizuki is a young child with long brown hair that falls to her waist and green eyes. She is the younger sister of Mana Ishizuki. At the beginning of the story, Koyori is staying in the same hospital as Aono Morimiya, but she is later discharged.

She is remarkably polite to everyone, even in situations where formal manners are not required. Despite her gentle disposition, there is a playful side to her relationship with her older sister. Whenever Koyori calls Mana by her given name instead of addressing her as her big sister, Mana gives her a light tap on the head, a recurring sibling joke that highlights their close and affectionate bond.

While in the hospital, Koyori goes out of her way to keep Aono company. The two become good friends during the time they spend together there, and Aono teaches Koyori how to make origami. This shared hobby cements their friendship and offers Koyori a creative outlet.

In the wider story, Koyori serves as a supporting presence who adds warmth and innocence to the group. She is a friend not only to Aono but also to Yorito Morimiya, and her caring nature helps bring out the gentler sides of those around her. Her most notable relationships center on her sister Mana and her hospital friend Aono, while her connection to Yorito remains friendly and uncomplicated.

Koyori does not undergo dramatic personal transformation during the narrative. Her primary contributions come from her unwavering friendliness, her desire to help others feel less lonely, and the small but meaningful ways she interacts with the main cast. The skill she most visibly develops is origami, which she learns from Aono and which symbolizes the bond they formed in the hospital.