Rei Uzumasa, aged 25, serves as a curator at a Kyoto museum where she applies her language skills professionally. She is the older sister of Moe Uzumasa, who admires her, and maintains a close bond with her. Rei's personal interests center on sake and tsukemono (Japanese pickles), and she is noted as "a bit of a drinker," sometimes staying out drinking until the last subway train. She shares drinks with Kyoka Ise, an assistant buyer at JR Kyoto Isetan.

Rei appears in expanded media and collaborations. A 2017 anime short featuring her was supported by a crowdfunding campaign that surpassed its initial goal. That same year, she was part of a themed café event at JR Kyoto Isetan, inspiring specific menu items.

In the mobile game *The Battle Cats*, Rei is a playable unit obtained via login rewards during collaborations. Her unit, "Rei Uzumasa & Cat," consistently performs critical hits. It evolves into "Rei Uzumasa & Cat CC," reflecting her established traits and emphasizing her late-night drinking. The game's lore references her museum career and family connection.