- Jake: Put-upon leader of the Animorphs.
- Marco: Jake's wisecracking best friend.
- Rachel: Jake's cousin, a gung-ho action girl.
- Cassie: Rachel's best friend; the Animorphs' animal-lover and environmentalist.
- Tobias: Trapped in hawk morph; the Animorphs' surveillance expert.
- Ax: An Andalite far away from his people; expert on alien technology.
- Visser Three: Power-mad, villainous Yeerk; leader of the Earth invasion.
- Erek King: Pacifistic android and the Animorphs' inside man.
- Animorphs - Earth's last hope: a group of teenagers with the power to morph.
- The Yeerk Empire - The race of alien parasites invading Earth.
- Andalites - The one great force opposing the Yeerks.
- Chee - Pacifistic dog-like androids who are allies to the Animorphs.
- The Ellimist - A god-like being who occasionally helps the Animorphs - but has his own inscrutable agenda.
- Crayak - An evil god-like being who opposes the Ellimist.
- Yeerk Peace Movement — a group of controllers who believe in voluntary cooperation between Yeerks and Humans rather than slavery.
The Ellimist, also known as Toomin, was a Ketran male who, through a series of extraordinary circumstances, became almost omnipotent, god-like being. He played a game throughout the universe with a similarly-powerful being known as Crayak, with Crayak aiming to destroy worlds and the Ellimist endeavouring to save them. He frequently interfered in the Animorphs' lives, sending them to possible futures and distant planets, so as to help defeat Crayak. The Ellimist also used the Animorphs' Chee friend, Erek King, in his endeavors. When he appeared to the Animorphs, he tended to take the form of an elderly male humanoid with glowing blue skin, similar to the typical wizard or wiseman archetype.