Captain Breakstone: Captain Breakstone is the President of the Court during the court-martial. He is a former submarine commander and is known for being a stern disciplinarian.
Jack Challee: Jack Challee is the prosecutor for the court-martial. He is somewhat anti-semitic, accusing Greenwald of using "smart Jew-lawyer tactics" to win the acquittal.
Ducely: Ducely is a wealthy newcomer to the Caine and works under Willie in communications. He admits that he is inept, but his failure is more related to his lack of interest than anything else. He departs the Caine when his mother, who owns a shipyard, arranges for him to have a soft war-time duty. He serves as a foil to Willie in that he represents the easy path Willie could have taken by having his uncle arrange for him to have a full commission in the Army.
Walter Feather : Walter Feather is the bandleader May becomes involved with when Willie rejects her.
Lieutenant Burt Gorton: Burt Gorton is the Caine's executive officer under de Vriess.
Barney Greenwald: Barney Greenwald is the lawyer who defends Maryk during the court-martial. In civilian life he was a talented lawyer, but he put his career on hold to become a fighter pilot. He is badly burned when the plane crashes. He is Jewish and is conscious of the fact that many in the Navy discriminate against Jews. Though reluctant to defend Maryk, he wins an acquittal. Later he is ashamed at having ruined Queeg's character during the trial. He accuses Keefer of being the true guilty party in the mutiny.
Harding: Harding joins the crew of the Caine at the same time as Willie. The pair bond in friendship high in the crow's nest when Willie lends Harding his cap so that he doesn't have to vomit on the sailors below.
Horrible: Everett Harold Black, known as Horrible, is one of the few men to continually serve on the Caine. He is the only man to die as a result of the kamikaze attack.
Roland Keefer : Roland Keefer is Tom Keefer's younger brother. He is another of Willie's roommates at midshipmen's school. He is stationed aboard an aircraft carrier and earns the Navy Cross for his heroic actions during a fire aboard ship. He dies as a result of injuries suffered in the fire.
Tom Keefer: Tom Keefer is Roland's brother. He is an educated and intelligent man. When Willie boards the Caine, he is the chief communications officer. He has a great contempt for naval service and only half-heartedly performs his duties. His real interest is writing a novel about the war, and he convinces Maryk that Queeg is insane. Keefer eventually becomes captain of the Caine, but he considers himself a failure for jumping overboard during the kamikaze attack and for his poor behavior during the court-martial.
Edwin Keggs : Edwin Keggs is one of Willie's roommates at midshipmen's school. He is a former high school algebra teacher. Keggs and Willie have very different experiences in the Navy, with Keggs assigned to a much more discipline-oriented commander and Willie assigned to the much looser Queeg. Keggs eventually becomes captain of the USS Moulton, another minesweeper.
Willis Seward Keith (Willie) : Willie Keith is the novel's main character. Willie is a graduate of Princeton in the arts. He comes from a well-to-do family in Manhasset, New York, and though he is educated, he has been spending his life playing piano at various nightclubs. Initially, Willie is a rather inept, bumbling sailor, who doesn't understand the Navy's way of doing business. He enjoys a good time and is somewhat of an elitist. However, his service on Caine and his exposure to the trials of war brings about a tremendous change in him.
Mrs. Keith : Mrs. Keith is a somewhat smothering individual. She truly loves her son and dreams of him having a distinguished career.
Dr. Keith : Dr. Keith is Willie's father. He is a very mild-mannered man and lets his wife run the family. He dies early in the novel. In some sense Dr. Keith believes that he traded the life he really wanted for a life of comfort. Willie believes his father felt he should marry May Wynn.
Captain Matson : At the admiral's request, Captain Matson arranges for Willie to remain in Hawaii so that Willie can play piano at the admiral's parties.
Steve Maryk : Steve Maryk is the Executive Officer aboard the Caine under Queeg. He is a former fisherman and wants to make a career in the Navy. Though he is an effective officer, he is doesn't approve of many of Queeg's actions; as a result, he is swayed by Keefer's assertion that Queeg is insane. Maryk documents Queeg's various offenses and at a moment when he believes the ship is in danger of sinking, he relieves Queeg of his duty. Maryk remains loyal to Keefer throughout the trial, never divulging Keefer's role in the mutiny. For a short time Maryk becomes commander of the Caine, but following his acquittal in the court-martial he is assigned to command a landing craft.
Marty Rubin : Marty Rubin is May Wynn's manager. Willie suspects Rubin of having an affair with May, but Rubin actually treats May fairly. He believes in and truly likes Willie and urges May to accept Willie's proposal. When Willie returns home after the war, Rubin takes him to find May.
Stilwell : Stilwell is the Caine's best helmsman. Unfortunately, he often bears the brunt of Queeg's wrath. He causes trouble for Maryk and Willie as they arrange to get him leave, against Queeg's orders, so that he can try to repair his broken marriage. Late in the war, he has a mental breakdown.
Captain de Vriess : Captain de Vriess is Willie's first captain on the Caine. Willie regards him as a poor commander, since his ship is in disarray and his men are slovenly and somewhat disrespectful of authority. Later, Willie reverses his opinion of de Vriess and concludes that de Vriess is a much better commander than Queeg.
Whittaker : Whittaker is an African-American steward's mate aboard the Caine.
May Wynn : May Wynn is a nightclub singer Willie meets while he is pianist at the Club Tahiti in New York. May's real name is Marie Minotti, and she is from a lower class, Italian family. She is very good looking but is somewhat self-conscious regarding her looks and her social status. She has a tempestuous relationship with Willie. After her rejection by Willie, she takes up with a bandleader; however, when Willie seeks her out after the war, it becomes evident that her passion for him is still alive.