Mayumi Uchida, often called Mayu-chan by her friends, is a student at Yokosuka Girls' Marine High School and serves as the starboard navigation controller on the training ship Harekaze. Her primary responsibility is ensuring the safety of the ship’s starboard side during navigation, a task she carries out with a calm and gentle demeanor. She is physically recognizable by her slightly tanned skin, a feature she insists is the result of deliberate tanning rather than her natural complexion, and she wears her dark brown hair in a braid tied with a blue-green bow on her left side. She also wears the standard school uniform, though with a longer skirt that falls around knee level.

Mayumi has a quiet and easygoing personality, rarely getting flustered even in stressful situations. Despite her composed exterior, she harbors a whimsical ambition to become wealthy overnight, frequently purchasing lottery tickets within the limits of her allowance. Her favorite saying is “Make a fortune at a single stroke,” reflecting her optimistic and slightly playful outlook on life. This dream of striking it rich is a recurring lighthearted motive that colors her interactions.

She is a childhood friend of Rin Shiretoko, another member of the Harekaze crew, and the two share a close bond. Mayumi often finds herself drawn into Rin’s schemes and adventures, following along with a patient, supportive attitude. Her role on the ship is primarily supportive; she works quietly behind the scenes, contributing to the smooth operation of the vessel without seeking the spotlight. She does not experience a dramatic personal arc over the course of the story, but she remains a dependable and steady presence among the crew, helping to maintain morale and order during the Harekaze’s tumultuous journey. Her notable ability lies in her competent performance as a navigation controller, where her attention to safety and calmness under pressure prove valuable to the ship’s operations.