Eri Kumagai is a high school student and online composer specializing in electronic music. Her calm, precise, and candid personality positions her as the straight man in comedic exchanges. Reserved by nature, she experiences discomfort in crowded settings and struggles with public speaking. She exhibits technical proficiency in computing and machinery, with hobbies spanning welding, cooking, and personal computer maintenance.

On her first day of high school, she encounters Haru Chitose, whose disruptive singing sparks initial friction. Despite Eri's reluctance, Haru discovers her musical compositions and persistently pursues collaboration. Their dynamic shifts from conflict to an uneasy partnership as Eri navigates Haru's energetic yet boundary-crossing behavior, including privacy invasions and public embarrassment. This culminates in Eri confronting Haru about these actions while recognizing their musical synergy.

Later, Eri transitions into a senior idol role within the unit H☆E☆S alongside Haru Chitose and Sarasa Ryuoh. She composes electronic-based music for the junior idol unit Ongaku Shoujo and supports the newer generation of idols, maintaining her foundational reserve and precision. Her narrative focus shifts from prominence in the original short film to a supporting role in the television series, where her composition work for Ongaku Shoujo recurs.

Eri's design features violet hair and eyes, often styled with hair antennae and occasionally a ponytail. Her theme color is blue.