Mayumi Uchida, known as "Mayu-chan," is the starboard navigation controller on the training vessel Harukaze, later designated Harekaze II. Her duty involves monitoring and safeguarding the ship's starboard side during operations. She possesses tanned, brown skin, a result of deliberate sun-tanning which she proudly displays.

Hailing from Namegata City, Ibaraki Prefecture, she shares a longstanding friendship with Rin Shiretoko and often gets drawn into Rin's habit of fleeing tense situations. Mayumi presents a calm and gentle demeanor but nurtures a persistent aspiration for sudden wealth. This fuels her habit of purchasing lottery tickets within her budget limits and partly motivated her enrollment in maritime training to become a Blue Mermaid, envisioning opportunities to acquire international lottery tickets during travels.

Academically, she excels in Politics, Economics, and Social Studies but struggles with Physics. Beyond her duties, she practices judo, holding a first dan rank with proficiency in ground techniques; challenges with standing techniques impede her advancement. She also holds an Assistant Surveyor qualification. Her preferences include tuna sushi as a favorite food, a dislike for barley rice, and a favored color of gold.

She maintains her navigation role aboard the Harekaze II during an inter-high tournament and a subsequent mission to counter a hijacked seafortress. During this operation, she performs her duties amidst combat scenarios, including the vessel's infiltration into the fortress and its subsequent escape.