Theodore is the tritagonist of the series and films. He is the youngest of the group.
Theodore is shy, cute, loving, sensitive, gullible, trusting, naive and innocent.
In the 80s cartoons, he is the youngest brother.
In the 60's, he is the middle child.
Theodore is a character that has a great interest in eating and cooking in the 1980’s and 1990’s television series. He is considered innocent and gullible and is portrayed to be the youngest. He cares deeply for Eleanor and often cares for her, being the most open about liking her of all the chipmunks. He is found to care deeply about his self-image trying on numerous occasions to attempt to become more fit or at the very least, trying to lose weight. Theodore loves for cooking is seen in episodes of the series such as Father's Day Muffins and Every Chipmunk Tells A Story and the special A Chipmunk Christmas.
Theodore is a chipmunk who helps out his brothers in their time of need. His instrument is the drum. His signature color is green. He is very gullible and innocent, always letting Alvin get to him with his schemes. He is shown in Alvin and the Chipmunks: The Squeakquel to be somewhat sensitive about his weight when the bullies poked his belly and butt and called him fat, even saying that he has "serious junk in the trunk." Alvin and Simon claw them up as a result, taking care and watching after their baby brother. He has a crush on Eleanor Miller, one of the Chipettes who has similar traits of Theodore, who shares his same interest, but not as gullible and she returns his feelings, but feels that he should stand up for himself more.
Theodore is commonly seen wearing a dark green top. He is the shortest chipmunk, about the same height as Eleanor, in all films and series except The Alvin Show in which he is taller than Alvin but shorter than Simon.
Before The Alvin Show, Theodore wore a pink vest with a "T" in it. After The Alvin Show was created, he wears a green sweater and black shoes.
In the Alvin and the Chipmunks series, Theodore wears a light green (sometimes light blue) sweater, and a pair of white and blue pattered shoes.
Theodore in the CGI films only wears a green hoodie. His appearance was modified to look more closely to a real life chipmunk.
In ALVINNN!!! and The Chipmunks, Theodore wears a green hoodie, blue jeans, and green and white shoes.
Theodore and Dave are extremely close. Theodore loves making things for Dave. For example, Theodore made a Get Well card for Dave when he was in the hospital in Alvin and the Chipmunks: The Squeakquel and he made a necklace for Dave in Alvin and the Chipmunks: Chipwrecked. Dave loves the things Theodore makes him and Theodore loves seeing his adoptive father happy.
Theodore loves Alvin because he is his brother. However, Theodore gets upset with Alvin sometimes. For example, when Alvin didn't come to the sing-off, Theodore ran away. But Theodore saves Alvin from a giant eagle and Alvin saves Theodore, too.
Theodore can always count on Simon to look after him. Theodore stays close to Simon, because he knows his brother will protect him, because they love each other.
Theodore and Eleanor love each other as much as they love food. Theodore knows that Eleanor will look after him. For example, when Zoe is crossing the bridge with Eleanor and Theodore, Eleanor covers Theodore's eyes for him.
- The origin of his name comes from Theodore Keep.
- In the 1960s The Alvin Show, Theodore was known for being the one who giggles. This changes into the 1980's for being the one who cooks. Theodore's character traits changed one more time for the modern CGI films to be the "fat" and sensitive chipmunk.