Kouko "Coco-chan" Nosa serves as the secretary aboard the training vessel *Harekaze*, third-in-command behind Captain Akeno Misaki and Deputy Captain Mashiro Munetani. Her duties include maintaining ship records, gathering critical intelligence during operations, and relaying tactical data to the bridge crew. She utilizes a tablet to process real-time information on opponent vessel specifications, environmental conditions, and damage assessments. She has brown hair styled in braids near her ears, each tied with pink ribbons. She is almost always seen wearing a white bonnet and sweater over her school uniform, a distinctive trait comparable to Mei Irizaki's tiger hoodie. Kouko exhibits a strong preference for fictional worlds over reality, frequently dramatizing situations or imagining elaborate scenarios, including adopting personas from Japanese gangster films. She struggles to maintain boundaries between imagination and actual events, often vocalizing theories or conjectures during daily interactions. This tendency forms the basis of her friendship with Wilhelmina Braunschweig Ingenohl Friedeburg, deputy captain of the *Admiral Graf Spee*, as they bond over shared cinematic interests. Their initial separation when the *Graf Spee* departs causes her visible distress, though she accepts Wilhelmina’s reassurance of future reunions. Her background develops across several installments. During the *Harekaze*’s initial voyage, she assists in evading false accusations of mutiny and manages intelligence during confrontations with infected vessels. She later reunites with Wilhelmina during Operation Perseus, where the *Graf Spee* aids in intercepting the rogue battleship *Musashi*. In the OVAs, facing potential dissolution of the *Harekaze* crew after the ship’s destruction, she initiates a petition campaign and organizes a festival to demonstrate solidarity upon learning rumors of reassignments. Her efforts culminate in the crew’s transfer to a new vessel, *Okikaze*, subsequently renamed *Harekaze II*, preserving their unit. The movie features her participation in a maritime festival and subsequent counter-piracy operation in Yokosuka harbor. She remains part of the crew’s coordinated response during the mission. Her surname Nosa combines the characters for "delivery" 納 and "sand" 沙, while her given name Kouko means "happiness" 幸 and "child" 子.