Yuuko Kanda is Sorata Kanda's younger sister, attending ninth grade during her appearances. She displays a pronounced brother complex, deceiving their mother to visit Sorata at Sakura Dormitory. Yuuko exhibits jealousy toward female residents like Mashiro Shiina and Nanami Aoyama due to their closeness with Sorata, extending to insecurity about her flat chest compared to theirs. Her competitive streak fuels direct confrontations with Mashiro, whom she considers a rival. She once referenced a cryptic "Plan C" strategy involving annihilation through cutting.

During a dormitory visit, Yuuko boasted about sleeping in Sorata's bed the previous year. When Mashiro mentioned doing the same, Yuuko reacted with intense shock, retreating into a self-described "Corner of Woe." Later, upon learning Mashiro and Sorata shared a bed again, she suffered a "Color Failure" moment—a dramatic loss of composure. Her interactions with Sorata's dorm mates consistently reveal possessiveness and unresolved sibling rivalry.