Mutsumi Sugimoto, known as Mu-chan, is a human third-grade student training to become a miko in Fuchigamori, a rural town where humans, yōkai, and gods coexist. Her life is cast into uncertainty by the unexplained disappearance of her father, leaving her plagued by profound loneliness and emotional turmoil. She frequently overhears conversations suggesting he may have been swallowed by the void—a dimensional rift between the living world and the afterlife—though his fate remains officially unresolved.

She develops a close bond with Jiro, a crow tengu serving as the town's spiritual guardian. He becomes a significant source of emotional support, partially filling the void left by her father. Despite her affection for Jiro, she struggles to articulate her feelings. Their interactions are often strained by his self-sacrificing tendencies and frequent apologies, which fuel her frustration and highlight her growing assertiveness.

Mu-chan possesses spiritual sensitivity, granting her perception of supernatural phenomena others miss. This includes sightings of entities like a daidarabocchi and the spirit of her deceased grandfather during Obon. Her sensitivity also manifests as an intuitive grasp of her father's probable fate, noted by Jiro as her ability to "see" the truth about his disappearance before verbal confirmation. She actively participates in community rituals, practicing a fire dance for local festivals and assisting Jiro in preparing offerings like Inari sushi for deities.

Her character development centers on processing grief and reconciling her father's loss. During the town's invasion by oni—demonic creatures linked to the void—and the collapse of kotodama (the spiritual power of words), she learns a critical revelation: oni are manifestations of memories of those consumed by the void. Utilizing kotodama, she transforms one of these oni into a shade resembling her father, achieving a symbolic resolution to her lingering sorrow and finding closure.