Luka Watanabe, widely known as Ruka, attends high school in Kochi Prefecture where she occupies a prominent social position. Her model-like physique and sparkling beauty naturally draw others to her, earning her the metaphorical status of the sun around which her peers gather. She possesses black hair and brown eyes, consistent with her Japanese heritage. Beyond her social standing, she actively participates in the school's brass band as an alto saxophone player.

Despite her widespread popularity, she forms a meaningful connection with the more reserved Suzu Naito. She confides in Suzu about her romantic feelings, specifically clarifying that her interest lies not in Shinobu Hisatake—Suzu’s childhood friend—but in Shinjiro Chikami, nicknamed Kamishin, a sports-oriented student. With Suzu’s assistance, she navigates this confession, resulting in a mutual romantic relationship with Kamishin.

Later, when Suzu and friends search for Kei—a vulnerable user in the virtual world "U" facing real-world abuse—Luka contributes critical observational skills. Analyzing background details in a video stream featuring Kei and his brother, she pinpoints their location as Kawasaki, Kanagawa Prefecture, near Tokyo. This deduction proves instrumental in guiding Suzu’s actions to intervene and protect the brothers.

Her involvement underscores a consistent willingness to extend support beyond superficial interactions, evident in her collaborations with classmates during urgent situations.