Tsumugi Tsutsumi is Shintarō Tokumitsu's classmate, recognizable by her chin-length light brown hair, brown eyes, and signature black hair ribbon. A brief childhood encounter, where she got lost visiting relatives and Shintarō helped her return, sparked her lasting romantic feelings. They only reunited nine years later in high school.

Outgoing by nature, Tsumugi becomes shy and hesitant specifically around Shintarō due to her unresolved crush. His obliviousness and her insecurity as the only completely normal human among his supernatural friends, including angels and yōkai, hinder her confession attempts. This fuels jealousy, particularly toward Towa who lives with Shintarō, and occasional envy toward Noel Izumi's physique, though Noel later reassures her.

Academically skilled and domestically adept, Tsumugi excels at studying and cooking, dedicating herself to baking. She uses these talents to engage Shintarō, offering to prepare meals. Her relationships extend beyond him: she maintains a childhood friendship with Shuuichi Seno, views Towa as a rival-turned-friend despite jealousy, and actively befriends Noel. With the yuki-onna, Tsumugi patiently challenges Noel's initially flawed understanding of friendship, insisting multiple relationships are possible; they eventually bond closely enough to use first names.

Her development includes episodic confrontations with jealousy, like worrying about competing against supernatural rivals, typically resolved quickly. In group settings without Shintarō, she often grounds interactions as the straight man with her human perspective. Steadfast in her affection, her inability to advance the relationship or alter Shintarō's preference for Towa leaves her a persistent but unfulfilled romantic interest.