Nagamori Mizuka is Kohei Orihara's childhood friend and classmate. She has long brown hair featuring spiky bangs, red eyes, and a slim, pale build. She is typically seen in her sailor school uniform, often accessorized with a ribbon tie or hair accessory. Kind, altruistic, and deeply fond of nature, Mizuka speaks with distinctive patterns, frequently adding "~da yo" and "~mon" to her sentences—a habit that prompts Kohei to affectionately nickname her a "Dayomonian." She consistently cares for Kohei, waking him every morning despite his heavy sleeping and enduring his regular teasing.

Mizuka keeps several cats at home, showing a strong affinity for strays that leads Kohei to call her a "cat whisperer." She exhibits a pronounced fondness for milk, drinking it with every meal, even alongside rice. She participates in the school orchestra club as a cellist, an activity tied to fragmented childhood memories involving Kohei. Her concern for Kohei extends to his social life, particularly worrying about him getting a girlfriend.

Her romantic feelings for Kohei stem from their first childhood encounter, where she offered him a comforting first kiss after he moved to his aunt's home. In her narrative route, Kohei makes a false romantic confession which she believes. Unwilling to reject her directly to avoid hurting her feelings, he instead escalates his mistreatment, culminating in an attempt to stage a near-rape experience using another person. He stops this plan before it progresses far, and Mizuka ultimately forgives him.

Within the Eternal World—a mystical alternate space linked to Kohei's traumatic past—Mizuka's childhood self occasionally appears as his guide. This connection relates to her fragmented memories of their shared history, such as recalling a "cello-playing boy" without full context. Her bond with Kohei proves pivotal in certain routes; her strong memories of him potentially aid his return from the Eternal World after roughly one year, though the mechanics depend on the specific relationship path.