Theodore is the tritagonist of the series and films. He is the youngest of the group and is best described as shy, loving, sensitive, gullible, trusting, naive and innocent.
In The Alvin Show, he is the middle child in terms of age. Theodore was commonly known for his giggling when he and his brothers were being mischievous.
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 retains his gullible and innocent nature, but looses the sensibility over his weight as seen in the CGI films that was unlike him in the 80s. He still loves to cook and eat. He is seen more as the younger sibling that is either taken advantage of or ignored. In the series, Theodore is seen multiple times with a teddy bear to keep him company as in Talking Teddy and Lil'T and is terrified of clowns as seen in Clowning Around.
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 zipped-hoodie which has a lighter shade of green on the hood, light blue jeans that are folded at the bottom, and green and cream colored shoes. He also has brown eyes and dark yellow fur.
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.
Theodore and Jeanette don't interact very often but when they do, Theodore tries to make Jeanette happy, such as in Jeanette Enchanted.
Theodore and Brittany do not interact very often.
|Theodore: Since when did Alvin feel like playing Around The World In 30 Days? - (The Chipmunk Adventure)|
|Theodore: I'll have two cheese enchiladas with extra sauce, a tostada grande, the quesadilla with a side of guacamole, two chillis with... - (The Chipmunk Adventure)|
|Theodore: And one quesa...dilla. - (The Chipmunk Adventure)|
|Theodore: Natasha! Natasha! Daddy's gone!! - (Meet Frankenstein)|
|Theodore: Dave! You gotta come get us! There's a monster after us, and we're hiding in Dragonland! - (Meet Frankenstein)|
|Theodore: The ants in France stay mainly in my pants. - (Meet Frankenstein)|
|Theodore: Guys! I think he made it angry! - (Alvin and the Chipmunks)|
|Theodore: But...we talk. - (Alvin and the Chipmunks)|
|Theodore: I thought my heart was gonna explode! - (Alvin and the Chipmunks)|
|Theodore: Hi, I'm Theodore. - (The Squeakquel)|
|Theodore: She's like a beautiful, green gumdrop. - (The Squeakquel)|
|Theodore: Please don't eat my brain! - (Chipwrecked)|
- 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.