- The Cat in the Hat – the mischievous, comedic narrator, and Jojo’s mentor.
- Jojo/the Boy – an imaginative and misunderstood young boy.
- Horton the Elephant – a compassionate and determined elephant.
- Gertrude McFuzz – a shy bird who falls in love with Horton and endeavors to help him.
- Mayzie LaBird – Horton’s vivacious yet lazy and self-centered bird neighbor.
- Sour Kangaroo – the ill-mannered leader of the jungle animals; she carries a Young Kangaroo with her.
- Mr. Mayor – the newly-elected mayor of Who-ville and Jojo’s father.
- Mrs. Mayor – the Mayor’s wife and Jojo’s mother.
- The Wickersham Brothers – three mischievous monkeys who serve the Sour Kangaroo as henchmen and make trouble for Horton.
- General Genghis Khan Schmitz – the warmongering leader of an army in the Butter Battle.