The Caine Mutiny is primarily about the changing character of Willie Keith. At the beginning of the novel, Willie Keith is immature, weak, and spoiled. Most of his actions are dictated by a desire to disconnect from his protective mother. Instead of putting his Princeton education to use, Willie becomes a lounge piano player. Willie is also unwilling to commit to his wonderful girlfriend because she does is not the rich, well-bred girl Willie's mother imagines for her son.
Willie slowly matures over the course of his Navy career. De Vriess immediately recognizes the potential beneath Willie's irresponsible, unmilitary exterior. By the end of the novel, Willie has become a man by dint of surviving the hardships of Queeg's command of the ship, the trials of war, and the risk of calamity and death. Just as Willie slims down because of the Navy food, he sheds his immaturity because of the Navy experience, and becomes a resilient, brave, and reliable man. Some would even call him a hero. Willie makes three major steps in his development. The first one comes in response to a letter in which his father observes that Willie has never had a major test in his life before the Navy, and how he fared would have a major bearing on the rest of his life. As a result of this letter, Willie determines to succeed in the Navy. The second step happens during the court martial, when Willie realizes that he acted to remove Caine from command out of personal dislike for the captain, not out of worry for the ship. He realizes that this was a mistake, and from then he is far more objective. The third and most important step on Willie's path to manhood occurs when the kamikaze strikes the Caine and Willie understands the one thing that he regrets and decides to rectify his fault.