Sasuke's sister lives in a forest deep within Japan's Shinano mountain region, sharing her home with her younger brother and their animal companions—deer, bears, squirrels, and monkeys. She nurtures and guides Sasuke, providing crucial support and information after traumatic experiences. Her role becomes critical when a monstrous salamander attacks, killing a mother deer among their animal friends. She then reveals the creature's true identity: the ancient demon Yakusha (Yasha-hime), transformed millennia ago by a wizard but now powerful enough to threaten Japan. This revelation drives Sasuke to seek magical training to confront the demon.

Her peaceful existence shatters when Yakusha's bandit allies assault their village, burning homes and capturing her. Sanada Yukimura, a prince whose own city fell to these bandits, rescues her. This encounter forges a connection between them, with Yukimura pledging continued aid to the villagers and implying a developing romantic subplot. Her kidnapping directly impacts Sasuke, compelling him to focus on defeating Yakusha while entrusting her safety to Yukimura.

Following her rescue, she stays within the community while Sasuke faces Yakusha in the final battle. Her name is identified as Oyuu in some sources, though this is not uniformly referenced.