Theodore is the youngest of the group. He is shy, cute, loving, sensitive, gullible, trusting, naive and innocent.
Theodore's age role has changed the least. In the 60's cartoons, he is the middle child, while in the 80's cartoons he is the youngest. In the CGI films his age is unknown.
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 naive, always letting Alvin get to him with his schemes. He loves to eat and is the youngest of the Chipmunks. He is shown in The Squeakquel to be somewhat sensitive about his weight when the bullies poked his belly and rear and called him "fat," even saying he has "Serious junk in the trunk." (This caused Simon and Alvin to claw them up in the end due to taking care and watching after their baby brother.) He also loves to cook and in the series of the cartoon and The Squeakquel. Examples of episodes of the 1980 TV series where Theodore cooks were Every Chipmunk Tells A Story and A Chipmunk Christmas. 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 he should stand up for himself more.
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 and he made a necklace for Dave in Chipwrecked. Dave loves the things Theodore makes him and Theodore loves seeing his adoptive father happy.
Theodore loves Alvin because he is his brother. But 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. A lot. 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 1960's 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 film to be the 'fat' and sensitive chipmunk.