Delton 4
A leader of Kreton's unidentified home planet, Delton 4 is an alien who enters the play at the end, after he is summoned by Ellen Spelding. He explains that Kreton is "morally retarded" and "was able to escape from his nursery.'' He apologizes for Kreton's actions and takes him home.

Kreton (kree-tahn)
Arriving on Earth from a distant planet in another dimension, Kreton is the "visitor" alluded to in the play's title. Like others who inhabit his home planet, Kreton never needs to eat, is immortal, and never has sex—immortality has negated the need for reproduction, and their culture has eliminated passion (sex for pleasure) as a societal evil. Through the powers of his mind, he is able to read the thoughts of others, create invisible force-fields, and cause objects to levitate. Kreton arrives hoping to witness a Civil War battle, but because of a navigational error, he lands in 1957. He invades the home of Roger Spelding and shows a great interest in the day-to-day lives of earthlings, calling them his "hobby." In order to create some excitement for himself, Kreton plays a prank that, he is sure, will cause a worldwide nuclear war. He sees the earth as a playground and hopes that a war will allow him to see the ways that “primitive'' humans behave. As a character, he is a curious mix of super intellectual and wide-eyed child.

Conrad Maybery
Ellen's boyfriend, Conrad, is a mild-mannered young fanner. A pacifist (who resents all forms of war), Conrad contrasts the political views of both Roger Spelding and General Powers. One of the play's comic scenes features Kreton attempting to provoke feelings of aggression within Conrad—which he eventually does, with a soldier whose thoughts about Ellen are broadcast by Kreton. Vidal uses Conrad to lampoon those who conveniently use pacifism as an excuse for their lack of direction and laziness.

General Tom Powers
General Powers is the Army commander assigned to investigate the spacecraft seen hovering over the Speldings' home. Complaining that he doesn't want this assignment and would rather return to his “Laundry Project'' (where he oversees the washing and drying of the military's uniforms), Powers suspects that Kreton may be a "hostile alien" who has been sent by a "foreign power" to begin an invasion of the United States. His character is frequently used by Vidal to poke fun at paranoid military attitudes, officious bureaucrats, and the prevalent fear of Communism found in 1950s America.

Ellen Spelding
Ellen is Roger's daughter, a bored college student who wishes for the opportunity to do "something important" like "save the world." She convinces Kreton to teach her how to perform some of the "tricks" with her mind that he can do, and she eventually uses these skills to contact Kreton's superior, Delton 4, and halt the war that Kreton has planned. She also focuses her romantic attentions on Conrad, a local farmer of whom her father disapproves, Ellen is the embodiment of youthful optimism and idealism. She is the only character that Vidal abstains from making look foolish.

Reba Spelding
Reba is Roger's wife and Ellen's mother. Her biggest concern in the play is that fact that Kreton's spacecraft has landed in her rose garden. In Reba, Vidal is making fun of the 1950s ideal of the perfect housewife, a domestic-minded woman obsessed with presenting the perfect image embodied by everything from her delicious apple pie to her beautiful, well-behaved children.

Roger Spelding
A news commentator who believes that UFOs do not exist, Roger's opinions are immediately reversed when his home is visited by an alien from another dimension. He is constantly thinking about the possible ratings his television show will receive when he "breaks" the story of Kreton's visit. When not thinking about his own potential fame, Roger is found scolding Ellen, his daughter, for her choice of a boyfriend. Roger represents the growing role that television has in American society. Already by the 1950s, television had become an important source of information, bestowing considerable status and power on those who controlled it. This explains Roger's eagerness to interview Kreton; his first instinct is not to protect his family from possible harm but to secure the television rights to the alien visitation.