Rui Nagata is a 15-year-old female student at Higashizaka High School, sharing classes with Daisuke Higashida and Miri Yanagiba. She possesses long blonde hair, initially worn in twin buns, which she later cuts shorter and ties into a single bun. Standing 156 cm tall, she has brown eyes. Kind and quiet by nature, Rui often misinterprets social situations or conversations.

She develops a crush on Daisuke Higashida, motivating her to agree when he asks her to befriend the frequently absent Miri Yanagiba. Though initially reluctant, Rui hopes this assistance will bring her closer to Daisuke. Her interactions with Miri blossom into a close friendship, allowing Rui to confide her feelings for Daisuke to Miri. This friendship becomes a source of emotional support for Rui, particularly after Daisuke begins dating Wagnaria floor manager Hana Miyakoshi. Initially viewing Hana as a rival, Rui eventually accepts and supports their relationship.

Her involvement with the Wagnaria restaurant increases as she becomes a regular customer, visiting to socialize with Miri and observe Daisuke and Hana. Rui transitions from harboring unrequited feelings for Daisuke to actively encouraging his relationship with Hana. She remains a supportive figure for Miri, aiding her social re-entry into school life. Rui's physical change—cutting her hair—occurs alongside this emotional growth, especially her reconciliation of feelings for Daisuke.