Isoroku is a large cat residing in Yokosuka Girls' Marine High School who is taken by Akeno aboard the Harekaze. He played a key role in preventing the Totalitarian Disease's rodent carriers from infecting the ship and rounding them up in boarding actions against the Admiral Graf Spee later.
Isoroku sports a dark yellow fur with brown stripes topside and a white underbelly. His eyes are a combination of yellow and brown.
Laid-back even under fire, Isoroku likes to spend most of its time slouching especially on the bridge's instruments. Nevertheless, he grows fond of Akeno and her classmates as he helps the boarding party round up the rodents on the Graf Spee later.
Before the events of Hai-Furi, Isoroku roamed the grounds of Yokosuka Girls' Marine High School, seemingly disinterested with everything around him. After meeting a friendly Akeno, however, he instinctively moved into the Harekaze, much to Mashiro's protest. He would be informally promoted to "Admiral" by Akeno.
As the infection claimed ship after ship, Principal Munetani described Isoroku's presence aboard the Harekaze as a "good wind blowing." As cats were immune to the effects of the virus, Isoroku managed to prevent the Harekaze's crew from falling victim to the disease. He captured two rodents on the Harekaze and an additional ten on the Graf Spee.
- Isoroku is named after Admiral Isoroku Yamamoto, commander-in-chief of the Imperial Japanese Navy for most of World War II who commanded key battles such as the Pearl Harbor Attack and the Battle of Midway.
- Over the course of the series, Akeno would informally promote Isoroku to "Grand Admiral." While no such rank exists within the Imperial Japanese Navy, Yamamoto did end his career as a Gensui or Marshal Admiral, the highest rank possible but honorific in nature.