Rui Nanakawa is the older twin sister of Tadashi Nanakawa. They attend different schools: Rui at Hibikino Private High School and Tadashi at Kirameki Private High School. Rui sometimes impersonates her brother, infiltrating Kirameki High in his place. Despite visible differences in their height and build, no one else at the school detects this deception; only the protagonist notices the switch. This situation echoes a similar premise from earlier entries in the series.

Her interests encompass anime, video games, and music. Her given name, Rui, combines the character "瑠" (ru), meaning "lapis lazuli," with "依" (i), meaning "rely on." The surname Nanakawa translates to "seven rivers."

Rui appears as a supporting character in the OVA "Tokimeki Memorial 4 Original Animation: Hajimari no Finder."