Riko Nagase is a second-year student at Kitauji High School, one academic year senior to Kumiko Oumae. She plays tuba in the school's concert band. Physically, she presents as a young teenage girl with a petite frame and slightly above-average weight. Her dark brown hair is styled in two low buns, complemented by brown eyes and relatively thick dark eyebrows. She wears the standard Kitauji High School uniform for second-year students.

Her personality is polite, soft-spoken, and friendly, maintaining a respectful attitude toward everyone. While generally amiable, she exhibits seriousness when necessary. Within the concert band's bass section, Riko shares tuba responsibilities with Takuya Gotou. Their professional collaboration evolves into a personal relationship as they begin dating after developing mutual interest. They maintain a close, supportive bond, though Riko prefers to keep their relationship private.

Appearing as a supporting character in "Our Promise: A Brand New Day" during Kumiko Oumae's second year, Riko actively participates in the concert band's rehearsals, training camps, and performances. Her dedicated presence contributes to the bass section's cohesion and the overall group dynamics.