While often seen in her school uniform, Kouko is almost never without her sweater and white bonnet similar to how Mei Irizaki is often seen with her tiger hoodie on. She has brown hair with a pair of braids by her ears. A pair of pink ribbons can be seen near the ends of these braids.
The world of fiction fascinates Kouko more than that of reality, which is why she often acts out her imagination of certain situations. She also loves to get into character, especially in Japanese gangster movies. She shares such an interest with Wilhelmina Braunschweig Ingenohl Friedeburg the two become friends over the course of the plot.
As secretary of the Harekaze, Kouko is third in command, just behind Akeno Misaki and Mashiro Munetani. She also gathers intel on anything relating to the Harekaze's situation such as its opponent details and specs as well as the environment it's fighting on and the damage control of the Harekaze.
She became friends with Wilhelmina after a shared interest in Japanese gangster films. But after the Admiral Graf Spee was saved from the virus, Wilhelmina rejoined her classmates and left for Seeadler for re-supply. While initially saddened by this, Kouko bid Wilhelmina goodbye, being assured that the latter would go on a trip to see each other again.
It wasn't long before they saw each other during Operation: Perseus. The Graf Spee participated in the operation to save the Musashi.
- The name Kouko means "happiness, good luck" (幸) (kou) and "child" (子) (ko).
- Kouko's surname Nosa means "delivery" (納) (no) and "sand" (沙) (sa).