Isoroku is a large obese 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, mainly the Harekaze and rounding them up in boarding actions against the Admiral Graf Spee later as he was immune to the disease.
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. However he becomes extremely energetic everytime he encounters rodent, chasing them until he catches it.
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 much to Mashiro's dismay as the cat "outranked" herself.
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. However he was killed in 1943 when his plane was shot down.
- 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.
- Interesntingly, his fur colour matches the cat hoodie that Mei Irizaki often wears.