Mutsumi Horikawa appears in the anime Houkago Mania Club, which is an adaptation of a visual novel. She is the older sister of Amy Horikawa, though her personality is almost the complete opposite of her younger sibling. Mutsumi is often seen wearing glasses, which give her an intelligent and somewhat bookish appearance, and she carries herself with a serious demeanor. Despite this outwardly square or plain impression, her approach to romance is earnest and thoughtful; she holds the belief that love is something to be carefully nurtured over time rather than something that happens spontaneously.

Within the story, Mutsumi is a literature-minded girl who enjoys creating picture books, a creative pursuit that reflects her introspective and diligent nature. Her motivations are closely tied to her sincere views on relationships and her desire to understand intimacy on her own terms. She navigates the activities of the club with a measured attitude, often serving as a contrast to more impulsive or outgoing characters. Her key relationship is with her sister Amy, as the two are pointedly different in temperament, which creates a dynamic of both tension and care. Mutsumi also interacts with other club members and the protagonist in ways that challenge her initial reserve, leading to gradual development where she becomes more open to new experiences while still holding onto her core values. Her notable abilities include her intellectual curiosity, her disciplined approach to romance, and her talent for writing and illustrating stories.