Kumasa is a central figure within the underground idol group ChamJam's fan community. He is a man in his mid-30s with a heavier build who wears glasses.

Kumasa left his job as a company employee to become a freelancer, enabling him to fully dedicate himself to supporting the idol Reo Igarashi. His loyalty to Reo began before ChamJam's formation. He witnessed the sudden end of Reo's previous idol group, an experience that cemented his belief in cherishing every moment of an idol's career due to its inherent uncertainty.

He acts as a mentor to the protagonist, Eripiyo, introducing her to idol fandom and guiding her through its culture. Despite their frequent, heated debates over their favorite idols, they share a strong and enduring friendship. Kumasa exhibits a level-headed, grounded approach to fandom that contrasts with Eripiyo's more extreme behavior, though he possesses similarly intense devotion. He is consistently polite, friendly, and well-regarded within the community.

His nickname "Kumasa" stems from the Japanese word for bear ("kuma"), reflecting his fondness for bears. The exact origin of the nickname—whether influenced by Reo's own affinity for bears or vice versa—remains ambiguous even to him. His loyalty to Reo is unwavering, demonstrated historically when he was her sole supporter in a prior group and promised to help her become number one. This pledge deeply moved Reo, highlighting the significance of his support during her less popular phase.

Kumasa embodies a pragmatic yet passionate approach to idol fandom, balancing fervent support with a stable, mature presence among ChamJam's fans.