Theodore Seville, or Theo Seville, or Theo, which Simon and Alvin or everybody calls him, is the youngest of the three chipmunk brothers who loves to eat, causing him to be chubby, much to Donald Walls' disgust. He is innocent and can get scared of things like Jungle Monster 4. In Alvin and the Chipmunks, he sleeps with Dave on Christmas Eve because he had a nightmare. In The Squeakquel, he gets scared of eagles, but later lose the fear. In Chip Wrecked,he chooses to watch a animated kiddie film called Hello Lolly. Alvin says that "It's for babies!" and switches it to the scary Muppet movie Jungle Monster 4, making Theodore scared. In the end, Eleanor makes Theodore a jungle monster costume out of a paper barf bag. Despite of his pacifist tendency, he can sometime be violent whenever somebody tries to provoke him.
Theodore has tan fur is chubby. He has green eyes and wears clothes that match his eye color.