Kōko Matsumoto appears as a supporting character in Episode 6. She attended high school with Miyako Hoshino, sharing membership in the same club. Witnessing Miyako's intense dedication to sewing and costume creation sparked an obsessive fixation in Kōko. Despite Miyako barely recalling their initial meeting, Kōko declares herself Miyako's "best friend" following a year of persistent stalking. Her daily routine involves walks in locations where she might see Miyako or observe her home, though rain prevents these outings. Kōko possesses extensive knowledge of Miyako's life, asserting she knows more about Miyako than Miyako herself.

Physically, Kōko has long brown hair and dark brown eyes. She frequently wears a white striped T-shirt with a pink top. Her hairstyle deliberately mirrors Miyako's high school appearance. Personality-wise, Kōko presents as cheerful and approachable, easily striking up conversations with strangers and offering help. However, any mention of Miyako triggers a dramatic shift to an intense demeanor that causes discomfort in others. This duality defines her interactions, particularly when her fixation surfaces.

Her relationships revolve intensely around Miyako. Kōko manipulates situations to gain entry to Miyako's home, telling Miyako's mother, Chizuru Hoshino, that she is Miyako's friend. This earns Chizuru's trust, leading Kōko to affectionately call her "mom." Kōko shares a close bond with her younger sister, Yuna Matsumoto; they often walk their dog together. Kōko named this dog "Miyako" after Miyako Hoshino. With Miyako's sister, Hinata Hoshino, Kōko bonds over their shared admiration for Miyako, prompting Hinata to address her informally as "Matsumoto." Initially wary, Noa Himesaka later views Kōko favorably after Kōko helps locate her lost puppy and sister. Noa occasionally contacts Kōko for information about Miyako.

Character development occurs gradually. Miyako remains uneasy, yet Kōko's consistent support fosters growing trust. By Valentine's Day, Miyako begins using Kōko's first name, indicating increased comfort. In the movie "Precious Friends," Kōko appears in a post-credits scene, enthusiastically displaying a photo of Miyako taken during the film's events, confirming her ongoing attention. Trivia notes include her October 16th birthday, her Libra zodiac sign, and the spelling "Kouko" on official merchandise. Her name combines "fragrant" (香, kō) and "child" (子, ko), while her surname Matsumoto means "pine tree" (松, matsu) and "origin" (本, moto).