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.

Rachel: She is also good in gymnastics and has an interest in fashion. She is Jake's cousin and is Cassie's best friend, although her warrior nature often conflicts with Cassie's pacifistic mindset. She and Tobias develop an attraction for one another during the series.

Tobias: Tobias has low self-esteem and was often bullied in school before becoming an Animorph. Tobias becomes trapped in a red-tailed hawk morph . During the series, he regains the power to morph, but his natural form is now as a hawk. Even though Rachel wants him to morph back to human and stay for more than the two hour time, but that would mean he would be out of the fight, so he remains as a red-tailed hawk. . He also develops a close friendship with Ax. He is also develops romantic feelings for Rachel.

Marco: Marco is the comic relief character in the series and Jake's best friend. He is also the one who coined the term "Animorphs." Marco is very pragmatic, often proposing the most direct solution to a given problem, even if it involves some sacrifices. He lives with his father, who is depressed over his wife's "death" in a boating accident. At first, he doesn't want to fight because he fears that his father can't survive without him. He changes his mind when he finds out his mother is still alive and the host body for Visser One, the leader of the Yeerk invasion.

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. 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.

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.

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.due to time spent as a human - the father of Tobias.

Tom : Jake's older brother, a Controller.

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.