Yoisho commands respect as the dependable and kind-hearted captain of a group of sailor mice. He maintains a calmer demeanor than the energetic protagonist Gamba, offering stability to the group. A past confrontation with the white weasel Noroi cost him his right eye, a significant physical trait. His steady leadership extends beyond seamanship, guiding the crew through challenges. He shares a longstanding friendship with the scholarly mouse Gakusha, indicating a history predating the main events, and family connections include a daughter named Yuri, referenced in adaptations where he is nicknamed Rusty. Yoisho joins the core mission to liberate Noroi Island after encountering Chūta, an injured mouse seeking aid against Noroi's tyranny. Throughout this journey, he contributes vital navigational expertise and combat experience. His prior encounter with Noroi adds personal stakes to the conflict, informing the group's understanding of the threat. Yoisho remains a steadfast presence within the central ensemble, consistently appearing among the seven primary mice undertaking the voyage.