Ruka Watanabe serves as a supporting character within the narrative. She attends the same high school as the protagonist and belongs to their social circle. Described as a popular student possessing a model-like figure, Ruka plays the alto saxophone in the school's brass band. Her personality naturally draws others toward her, earning her the metaphorical designation of "the sun."

She develops romantic feelings for her classmate Shinjiro "Kamishin" Chikami, who is passionate about canoeing. Initially, her friend Suzu mistakenly believes Ruka harbors feelings for Shinobu Hisatake, Suzu's childhood friend. Ruka confides in Suzu about her true feelings for Chikami. Following this confession, Suzu assists both Ruka and Chikami in navigating their mutual shyness, facilitating their eventual romantic connection.

During the story's climax, involving the search for an abused teenager named Kei and his younger brother Tomo, Ruka collaborates with Chikami. Using contextual clues gathered online and within their investigation, they deduce the brothers reside in Kawasaki, Kanagawa Prefecture, near Tokyo. This deduction proves crucial in locating Kei and Tomo before authorities could legally intervene in their domestic situation.