Ruka Watanabe, nicknamed Ruka-chan, is a popular student at Suzu Naito's high school, admired for her striking model-like figure and magnetic, sparkling presence that draws others in, earning her the moniker "the sun" among peers. She plays alto saxophone in the school's brass band and leads the music club with skill, balancing academics and extracurriculars effortlessly. Beneath her confident exterior, Ruka conceals personal vulnerability, particularly in expressing romantic feelings.

She confides exclusively in Suzu about her crush on classmate Shinjiro "Kamishin" Chikami, revealing her shyness and reluctance to approach him. With Suzu's encouragement and practical help, Ruka and Kamishin eventually confess their mutual feelings, resolving this personal subplot. Ruka demonstrates perceptiveness and empathy, noticing Suzu's emotional distress and offering crucial support during key moments.

In the investigation to uncover the Dragon's real-world identity and location, Ruka collaborates closely with Kamishin to decipher contextual clues from online streams and real-world data. Their joint deduction successfully pinpoints Kei and Tomo's hometown as Kawasaki, Kanagawa Prefecture, near Tokyo. Ruka later actively participates alongside Shinobu, Kamishin, and the choir teachers in urging Suzu to publicly reveal herself as Belle to gain Kei's trust, underscoring her integral role within the friend group.