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 to have nightmares (as well as frequent fears such as eagles and jungle monsters) and often sleeps with Dave in one of the first two films. 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 going as far as saying that he has "serious junk in the trunk." Alvin and Simon got furious upon seeing their younger brother get picked on because of his weight and clawed 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 loses 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 (Talking Teddy and Lil'T). He is also terrified of clowns (Clowning Around) and does not always chew his food (Curiosity).
Theodore is commonly seen wearing a dark green top. He is the second shortest chipmunk, slightly taller than 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 green sweater, and a pair of white and blue pattered shoes.
Theodore, in the CGI/live-action films, has light brown/dark gold fur and green eyes. He 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 has dark yellow or light brown fur and brown eyes. His pajamas consist of a matching set of a white and green striped long-sleeved shirt and pants.
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 necklace that Theodore made for him and Theodore loves seeing his adoptive father happy.
Theodore loves Alvin because he is his brother. However, Theodore gets upset with Alvin sometimes, though they tend to get along once in a while and work together, as in the CGI films. Theodore is usually dragged into Alvin schemes.
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. Though Simon gets frustrated with Theodore sometimes because Theodore quite doesn't follow or listens to what Simon says, But besides that they get along most of the time.
Theodore and Eleanor are the most open about their relationship. They spend lots of time together and enjoy each others company. Theodore has given Eleanor a necklace. They are shown to be married as adults.
Theodore and Jeanette are friends, but they don't interact very often. In The Brunch Club, Jeanette stood up for him when Alvin, Simon, and Brittany teased him about his weight. In Jeanette Enchanted, he gives a witch hat to Jeanette on her birthday to extend her "magical powers."
Theodore and Brittany are friends, but they do not interact very often. She seems to care about Theodore such as in Dragon Dad when she didn't want to take the dragon egg from Theodore but did so not wanting him to be upset when it failed to hatch.
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/live-action films to be the "fat" and sensitive chipmunk.
In the CGI television series, Theodore's middle name is Jameson, as stated in Lil'T.