Kouko Nosa, nicknamed Coco-chan, occupies the position of secretary aboard the destroyer Harekaze, standing as third-in-command behind the captain and deputy captain. Her duties encompass collecting tactical intelligence on opposing vessels, evaluating environmental conditions, and overseeing damage control reports during combat. She relays vital data to the bridge crew to guide strategic choices.

She consistently sports a white bonnet and sweater over her school uniform, with brown hair styled in braids secured by pink ribbons near the ends.

Kouko displays a marked preference for fictional worlds over reality, frequently dramatizing situations or mimicking characters from Japanese gangster films. This habit obscures her distinction between fantasy and actual events. She develops a significant friendship with Wilhelmina Braunschweig Ingenohl Friedeburg, deputy captain of the Admiral Graf Spee, fueled by their shared passion for such cinema.

After the Harekaze's damage and decommissioning, Kouko receives sealed orders from the principal via Captain Akeno Misaki, destined for each crewmate. Whispers of the crew's disbandment and reassignment to other ships provoke her anxiety over separation. She spearheads a petition to preserve the crew and orchestrates a festival to galvanize support and gather signatures. Her campaign results in the crew reuniting aboard the newly assigned Okikaze, later re-designated as the Harekaze.

During Operation Perseus, targeting the interception of the battleship Musashi, Kouko reunites with Wilhelmina, whose vessel joins the mission. She later features in the film continuity during subsequent naval operations.