Jake: Jake is the leader of Animorphs. Despite being a natural at it, he's very reluctant to lead the team, though he ultimately accepts the role. The war takes on a deeper meaning for Jake when he finds out that his older brother Tom has been infested by a Yeerk, it is also one of the only things that holds him together throughout the series; if not for that he would probably crack from all the pressure and life-taking decisions. He is the only member of the team to have some relationship with all of the other human members prior to the war: Jake and Rachel are cousins. He and Marco have been best friends since early childhood. He and Cassie have always had an attraction to each other. He is one of the few people who acknowledges Tobias and treats him with kindness.

Cassie: Cassie lives on a farm with her parents,who are veterinarians. She is the most knowledgeable about animals, and she is also an environmentalist. She is best friends with Rachel, although their personalities and styles are polar opposites. She loves nature and animals. Cassie is also an estreen, the Andalite term for someone who is naturally talented at morphing.

Aximili-Esgarrouth-Isthill (Ax): Aximili-Esgarrouth-Isthill is an Andalite who is thought to be the only survivor of a bloody battle that took place in outer space. His ship has crash landed on Earth. He is the younger brother of Prince Elfangor-Sirinial-Shamtul, who gave the Animorphs their morphing ability and died shortly after. He considers Jake his 'prince' (similar to an earth military commander) and has a strong friendship with Tobias. The other Animorphs call him "Ax" because it is easier to pronounce.

Visser Three: Born as Esplin 9466 Primary and later known as Visser One, he is the leader of the Yeerk forces on Earth and the main antagonist of the series. He inherited the planet from Edriss 562, who was Visser One at the time. Sadistic and cruel, with a penchant for torture, he almost exclusively inhabits the body of a fully-grown Andalite warrior, Alloran-Semitur-Corrass, and is the only Yeerk to have an Andalite host. He also has the Andalite ability to morph, and is shown throughout the series to have acquired some exceptionally powerful morphs from many different planets. He later gets promoted to Visser One.

Ellimist: A seemingly all powerful, God-like being who intermittently helps the Animorphs, including giving Tobias the ability to morph again. He tries to refrain from direct involvement in the war to avoid antagonizing his evil counterpart Crayak because combat between the two of them causes destruction on a galactic scale. He subtly orchestrated many of the key events of the series. In The Ellimist Chronicles, it is revealed that he was once a simple member of an avian race who, through a number of experiences, became a drastically different being.

Crayak: The arch-enemy of the Ellimist who seeks absolute control over all intelligent life. He develops a personal vendetta against Jake after Jake ruins his soldier species, the Howlers. The Ellimist claimed that he originated from another galaxy and was driven out by another power even greater than he was; The Ellimist Chronicles details his early conflicts with the Ellimist prior to their evolution into beings of seemingly limitless power.

Elfangor-Sirinial-Shamtul - Elfangor is the first alien the Animorphs meet in the series. He gives them the power to morph minutes before his death. His adventures are highlighted in the book called The Andalite Chronicles, which takes place before the main Animorphs series. He was a war prince, and the older brother of Aximili-Esgarrouth-Isthill. He was the sworn enemy of Visser Three, and - due to time spent as a human - the father of Tobias.

Auxiliary Animorphs: These were teenagers recruited from hospitals and rehabilitation centers where they were being treated for chronic diseases or learning to live with disabilities. The ability to morph restored some of the teenagers' bodies to perfect health, but failed to when the diseases or conditions were genetic.

David: A boy who joins the Animorphs after learning their secret. The Animorphs trust him at first, giving him the ability to morph and fight with them rather than kill him, but he betrays them. Ultimately, they are forced to trick him into morphing into a rat and trap him permanently in the morph, leaving him stranded on an island. He returns later in the series, eventually asking Rachel to kill him. Whether Rachel kills David or not has never been established.

Drode: The Drode is an alien creature, described as being similar to a very dark purple dinosaur with wrinkled, pruny skin, and an oddly humanoid face. Little is known of the Drode, but it serves Crayak and usually shows up when he does.

Erek King: Erek King is a member of the Chee, a pacifistic android race created by an alien race called the Pemalites. After the destruction of the Pemalites at the hands of the Howlers, Erek, along with the rest of the Chee, escaped to Earth. They have lived on the planet for thousands of years, using incredibly advanced holographic technology to pass as human.

The One: An alien being who assimilates other beings; he makes a deal with the defeated Yeerks. He is the final antagonist of the series

Tom Berenson: Jake's older brother, a Controller. His original Yeerk is actually promoted and meant to receive another host early in the series, but ends up in Jake instead; it subsequently dies, while Tom receives another Yeerk. This Yeerk eventually stole the cube that provided the morphing power to the Animorphs and delivered it to his fellow Yeerks, and was later subsequently killed by Rachel.

Aldrea-Iskillion-Falan - Aldrea is the daughter of Prince Seerow, a main character in Animorphs #34: The Prophecy and the heroine in The Hork-Bajir Chronicles. She has a strong personality mixed with a desire to do the right thing, not necessarily what is easy. Born an Andalite, her father Seerow gave the Yeerks' technology that they used to leave their home planet and begin their quest to dominate the universe. Her family was eventually sent to the Hork-Bajir home planet, where she bonded with an unusually intelligent Hork-Bajir named Dak Hamee. Aldrea obtained the ability to morph shortly after the technology was developed, and used it to fight the Yeerk invasion of the Hork-Bajir world after her family was murdered. She and Dak led the resistance, and a strong bond developed between the two of them. She eventually, though accidentally, became a Hork-Bajir female permanently, and she and Dak fell deeply in love, married and had a son named Seerow. Despite their resistance efforts, they were both eventually killed and their son was infested by the Yeerks, though his line later gave rise to the leaders of the free Hork-Bajir on Earth. A copy of Aldrea's memories and personality made before her death was later temporarily implanted in Cassie in order to gain access to a cache of Yeerk weapons stolen and hidden by Aldrea and the Hork-Bajir resistance on the Hork-Bajir homeworld.

Alloran-Semitur-Corrass - Andalite host body of Visser Three. Alloran was an officer under Prince Seerow, who relieved his superior of duty after a force of Yeerks stole Andalite ships and left their planet thanks to technology provided to them by Seerow. Years later, having been promoted to the rank of War Prince, Alloran was placed in command of the Andalite forces sent to fight the Yeerk invasion of the Hork-Bajir homeworld. Unfortunately, the desperate situation on the planet led Alloran to decide that wiping out the Hork-Bajir with a virus was a better alternative than allowing them to be enslaved, and countless members of the species died as a result. Alloran was subsequently disgraced, and ended up serving on the crew of the same ship that Elfangor was assigned to as a junior officer. A mission that the pair went on with Elfangor's peer Arbron and a pair of humans led to Alloran's infestation by the Yeerk who would become Visser Three, who used Alloran's morphing abilities for many vile purposes throughout the years. After an attempt by Ax to kill Visser Three, Alloran was briefly left free to communicate with him, and encouraged him to continue the fight. Eventually he was freed from his infestation.

Hork-Bajir:

Toby Hamee - Leader of the Free Hork-Bajir on Earth; a Seer or exceptionally intelligent member of the Hork-Bajir race.

Jara Hamee - Toby's father and the descendant of Dak Hamee, a Hork-Bajir Seer, and Aldrea, an Andalite-turned-Hork-Bajir who led the Hork-Bajir resistance against the Yeerks. He was killed in action during the final battle.

Ket Halpak - Toby's mother and Jara Hamee's wife.

Howlers: A race of killers artificially created by Crayak and deployed as shock troopers against the inhabitants of various worlds. Seven of them were selected by Crayak to be pitted against the Animorphs and Erek the Chee in a contest between the Ellimist and Crayak concerning the fate of an offshoot of the Yeerk race who had found an alternative means of living. Jake eventually discovered that the Howlers were children, too young to understand what they were doing and seeing it all as a game, and the Animorphs managed to infect their collective memory with a flashback of Jake and Cassie kissing. The Howlers were thus ruined as a soldier race.

Hedrick Chapman: Vice-principal of the Animorphs' school and a Controller.

Yeerks: The main antagonists, described as small, slug-like parasites that enter various organisms' brains to control their behavior. They are dependent upon Kandrona rays to survive, and must leave their hosts every three days in order to enter a Yeerk pool where they may absorb these rays.

Edriss 562 - Edriss 562 is a Yeerk that controls Marco's mother Eva. For most of the series, her rank is Visser One. She is the subject of the novel Visser, which describes her rise to and dramatic fall from power. She is the highest ranking of all the Yeerks in their military, and is only surpassed in importance by the Council of Thirteen.