Thomas Fowler is a British journalist in his fifties who has been covering the French war in Vietnam for over two years. He has become a very jaded and cynical man. He meets Alden Pyle and finds him naïve. Throughout the book Fowler is often caught in lies and sometimes there may be speculation that he is lying to himself. Fowler's relationship with Vietnamese woman Phuong often fuels the conflict in the story, especially between Fowler and Pyle. Fowler is also used as a metaphor to describe the character.
Alden Pyle is the "quiet American" of the title. A CIA agent working under cover, Pyle is thoughtful, soft-spoken, intellectual, serious, and idealistic. He comes from a privileged East Coast background. His father is a renowned professor of underwater erosion who has appeared on the cover of Time magazine; his mother is well respected in their community. Pyle is a brilliant graduate of Harvard University. He has studied theories of government and society, and is particularly devoted to a scholar named York Harding. Harding's theory is that neither Communism nor colonialism is the answer in foreign lands like Vietnam, but rather a "Third Force", usually a combination of traditions, works best. Pyle has read Harding's numerous books many times and has adopted Harding's thinking as his own. Pyle also strives to be a member of this "Third Force". US military counter-insurgency expert Edward Lansdale, who was stationed in Vietnam 1953–1957, is sometimes cited as a model for Pyle's character.
Phuong, Fowler's lover at the beginning of the novel, is a beautiful young Vietnamese woman who stays with him for security and protection, and leaves him for the same reason. She is viewed by Fowler as a lover to be taken for granted and by Pyle as a delicate flower to be protected, but Greene never makes clear which, if either, of these views is actually the truth. Pyle's desire for Phuong was largely interpreted by critics to parallel his desire for a non-communist south Vietnam. Her character is never fully developed or revealed. She is never able to show her emotions, as her older sister makes decisions for her. She is named after, but not based on, a Vietnamese friend of Greene's.
Vigot, a French inspector at the Sûreté, investigates Pyle's death. He is a man torn between doing his duty (pursuing Pyle's death and questioning Fowler) and doing what is best for the country (letting the matter go). He and Fowler are oddly akin in some ways, both faintly cynical and weary of the world; hence their discussion of Blaise Pascal. But they are divided by the differences in their faith: Vigot is a Roman Catholic and Fowler an atheist.