- The American son of a clergyman who is on the way to becoming a renowned psychologist when he falls in love with Nicole and marries her. Dick is extraordinarily charismatic and graceful at the start of the novel, but eventually falls to his ruin.
- Born Nicole Warren, the beautiful heiress to a wealthy Chicago magnate, Nicole was sexually abused by her father and suffers from mental breakdowns as a result. She falls in love with Dick at first sight, and the two enjoy an extravagant and turbulent life in Europe at the center of a sophisticated group of friends.
- The beautiful young movie star, born in America but educated in France, who falls in love with Dick at first sight and contributes to his dissipation through their affair.
- The half-American, half French-mercenary soldier who is in love with Nicole. He ends up taking her away from Dick toward the end of the novel.
- A dear friend of Dick's and a once brilliant musician who drinks away his career. He is ultimately killed at a speakeasy in New York.
- The lovely and kind wife of Abe. She marries the wealthy Conte di Minghetti after Abe passes away.
- The anglophile older sister of Nicole.
Franz Gregorovius or Dr. Gregory
- Born into a line of fine psychologists, he opens a clinic with Dick.
- The wealthy father of Nicole and Baby. He sexually abused Nicole and then placed her in a mental clinic in Zurich.
- The once obnoxious American intellectual with an inferiority complex who establishes himself as a prize author following his duel with Tommy Barban.
- The obnoxious, self-important wife of Albert McKisco.
- A young Yale graduate and acquaintance of Rosemary. He gossips with Dick about Rosemary and helps rescue Dick from an Italian prison.
- The twice-widowed mother of Rosemary Hoyt.
- A British snob whom Dick insults gravely and then rescues from prison.
Conte di Minghetti
- The second husband of Mary North, whom Dick and Nicole manage to insult terribly.
- An American filmmaker in France toward whom Rosemary maintains a momentary fascination.
- A heavy, effeminate friend of Albert McKisko's, he takes Rosemary to watch the duel.
- A black man who is killed in Rosemary's room in Paris on account of Abe North's careless drinking.
Lanier and Topsy
- Dick and Nicole's son and daughter.
- An American woman who snubs Nicole at the train station before killing an Englishman with a revolver.
- The Russian Prince whom Tommy Barban rescues from Russia.
- A psychologist who initially handles Nicole's case and urges Dick to terminate his relationship with her.