Eri Kumagai is a 16-year-old high school student from Saitama City, Saitama Prefecture, born September 3. She possesses violet hair, violet eyes, and stands 158cm tall. Her personality is candid, calm, and precise, often acting as the straight man in comedic exchanges. She shrinks from crowded spaces and exhibits shyness, especially in social settings.

Initially struggling with public speaking at school, she sang softly during music class. A turning point arrived with Haru Chitose, an eccentric and outgoing transfer student. Despite Eri's frustration with Haru's intrusive actions—like invading her privacy and uncovering her secret musical compositions—their relationship shifted. Eri creates electronic music privately, an endeavor exposed when Haru discovered her recordings and identified her as the online composer "noconoco." This revelation spurred collaboration, though Eri initially resisted partnering due to reservations about Haru's singing and personality.

Professionally, Eri belongs to the senior idol unit H☆E☆S alongside Haru Chitose and Sarasa Ryuoh. She composes electronic music for the larger idol group Ongaku Shoujo. Within H☆E☆S, she provides musical direction and balances Haru's impulsiveness. Key growth moments include performing her compositions live, such as singing a new song solely with Hiyo Yukino, demonstrating increasing confidence.

Interpersonally, Eri experiences nervousness during close encounters with Haru, like small talk on a bed or when Haru attempts to touch her waist, causing Eri to react shakily. Her dynamic with Haru evolved from tension to collaboration as Haru's persistent admiration for Eri's music gradually pierced Eri's reserved exterior. This relationship is central to Eri's journey from isolation to shared creative participation.

Her hobbies and interests include welding with a soldering iron, cooking, working with machines, and using personal computers. These technical and creative pursuits complement her methodical, detail-oriented approach to music composition.