- Moon-Watcher: an Australopithecus africanus man-ape who lived in dry equatorial Africa, circa 3,000,000 BP. He, his tribe, and his species were faced with extinction, but for the intervention of a monolith which imbued him and his fellows with the beginnings of higher intelligence. Evolution then took its course.
- Dr. Heywood Floyd: Astronomer and chairman of the "National Council of Astronautics," who is flown from the Earth to the Moon on a secret mission concerning the Tycho monolith, or TMA-1, a second monolith which has been discovered on the Moon, and excavated by an American geological survey team from Clavius Base. In Floyd's presence, the Tycho monolith, being struck by sunlight for the first time in three million years, sends an extremely powerful energy burst in the direction of Saturn.
- David Bowman: First Captain of the Discovery, an American spacecraft which had previously been intended to visit the Jupiter system, but which has recently instead been charged with exploring Saturn. Unbeknownst to Bowman and his deputy Frank Poole, the true objective of the mission is reconnaissance of Saturn space, implicated by the above energy signal direction, where it is thought that further clues to the aliens' intent may be found.
- Frank Poole: Bowman's deputy. Bowman and Poole routinely toggle shifts and roles in order to maintain a continuous, waking presence on board the Discovery. Poole is attacked and seemingly killed by HAL while performing EVA maintenance.
- HAL 9000: the Discovery's intelligent, on board ship's computer, capable of monitoring all ship functions, and holding conversations and maintaining relationships with human beings. HAL malfunctions, killing all crew members with the exception of Bowman; this was a result both of HAL being entrusted with the true objective of the Discovery's mission, and also of being charged with keeping it a secret until the appropriate time. These orders conflict with HAL's basic purpose, which is the accurate and complete communication of information to humans.
Over the course of the novel, several minor characters either appear very briefly or are named only in passing, including other man-apes, spaceflight staff, lunar station security, and Discovery crew members. Among the novel's minor characters, some of the more consequential are listed below (often having direct film equivalents, or else being recurring characters in the Odyssey novel series).
- One-Ear: the leader of the "Others," a rival tribe of Australopithecus africanus man-apes, thus the enemy of Moon-Watcher and his tribe. Aside from his eponymic disfigurement, One-Ear is described as being the same age and size as Moon-Watcher, but in "poorer shape." Inter-tribal confrontations reveal a leadership style characterized by concealing his fear behind either screeching bravado or "exaggerated dignity." When faced with his most lethal opponent, One-Ear appears to be either "too brave or too stupid to run." None of the Others, including One-Ear, are depicted as being evaluated or transformed by a monolith.
- Dr. Dimitri Moisevitch: a good friend of Floyd's (and citizen of the rival U.S.S.R.), who diplomatically presses Floyd about the Americans' "outbreak" cover story (intended to keep outsiders away from the discovery of the Tycho monolith) while both are aboard Space Station One, a stopping-point between the Earth and the Moon. In the film, Moisevitch's role is carried out by a character of a different name, Dr. Andrey Smislov, while the character of Moisevitch as-such makes appearances throughout the rest of the Odyssey series and in the opening scene of the 2010 film. Moisevitch, like Floyd, is written as a cagey and worldly bureaucrat.
- Ralph Halvorsen: Administrator of the Southern Province and leader of Clavius Base, Halvorsen represents American interests on the Moon, and receives Floyd at Clavius to gather information about the Tycho monolith.
- Whitehead, Kaminski, and Hunter: a trio of astronauts who are put aboard the Discovery in hibernation, in order to conserve resources. In the event of the deaths of Bowman or Poole, the hibernating astronauts are to be revived in the order given. Consequently, upon Poole's presumptive death, Bowman partially revived Whitehead until HAL depressurized the Discovery, killing the three while Bowman escaped to a secure area. In the film, HAL kills the trio by simply shutting off their life support functions while Bowman is on EVA in an effort to recover Poole's body; in the film, the trio are instead styled as Kimball, Kaminsky, and Hunter.
- Dr. Chandra: briefly cited by HAL during his de-activation as being his first instructor, who taught him to sing "Daisy". In the film, this initial instructor is instead named as "Mr. Langley"; however the character of Dr. Chandra features throughout the rest of the Odyssey franchise, especially in the 2010 book and film.
- Anna, Betty, Clara: the namesakes of the Discovery's three EVA pods. In other films and novels of the franchise, these pods are given different naming conventions. Betty is the pod which, under HAL's control, presumptively causes Poole's death.