Yui Mikoze is an 18-year-old third-year student at Nagarekawa High School who works as the performer for the local mascot, Uogokoro-kun, to promote her town and its businesses. She has brown hair tied in a high ponytail with bangs hanging over her eyebrows and distinctive red eyes. Exceptionally athletic and strong, she performs various acrobatic feats despite the heavy mascot costume's constraints.

She appends "~su" to her sentences as a verbal tic. Outside the costume, she exhibits a degree of shyness in social interactions, often blushing luminescently or going into a meltdown when others get too close, which is a stated reason she prefers performing in costume. She plans to use her locodol savings to enroll in voice acting school.

Mirai Nazukari is later hired as a substitute performer to help manage her workload. In an OVA, Yui trains her stamina and movement in a river with Mirai, using the water's resistance. She and the other Nagarekawa Girls once gave a tour of their town to the Awa Awa Girls from Tokunami City, showcasing local dining spots and the public pool to demonstrate community spirit.

In a crossover, Uogokoro-kun guest-starred in another series as a contestant in three games—Soccer, Karuta, and Sumo Wrestling—for an annual Yuru-Kyara Grand Prix, winning the rivalry. Her relationships include being a co-worker and friend to Nanako Usami and Yukari Kohinata. She is also noted to have a substantial bust line.