Mayumi Kino is a young woman around eighteen years old who serves as a crew member of Blue Submarine No. 6. She is described as headstrong, self-righteous, and driven by a powerful desire for revenge. Her family was killed by the forces of the rogue scientist Zorndyke, and this personal loss shapes her entire motivation. She joins the Blue Fleet with the primary goal of avenging her family, and this quest for vengeance often makes her strict with both herself and others. In the story, she is tasked with recruiting the reluctant former pilot Tetsu Hayami to the crew of Blue 6, approaching him with determination despite his cynical and disengaged attitude. Her role is that of a committed and passionate member of the submarine crew, pushing forward the mission to confront Zorndyke. She holds a deep sense of righteousness and believes in the cause of humanity's survival. Over the course of the narrative, her interactions with Hayami and the harsh realities of war may challenge her single-minded focus on revenge, suggesting a potential for growth beyond her initial motivations. She has no notable supernatural or combat abilities; her strength lies in her resolve and her role as a member of the Blue Fleet.