Theodore Seville is the overall tritagonist of the series and films. He is the youngest of the The Chipmunks and is best described as shy, sweet, kind, caring, loving, sensitive, gullible, trusting, naïve and innocent.
Theodore is a chipmunk who helps out his brothers in their time of need. His primary 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 the first movie. He is shown in Alvin and the Chipmunks: The Squeakquel to be somewhat sensitive about his weight when the bullies poke his belly and butt and call him fat, even going as far as saying that he has "serious junk in the trunk." Alvin and Simon get furious upon seeing their little brother get picked on because of his weight and claw them up as a result, taking care and watching after their little 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 innocent and gullible nature, but loses the sensibility over his weight as seen in the CGI films that was unlike him in the 1980s. He still loves the culinary arts. He is seen more as the younger brother that is either taken advantage of or ignored by both of his brothers. In the series, Theodore is seen a number of times carrying a teddy bear (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 dark 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 sneakers. In the first 5 seasons, he has black eyes, and in the Chipmunk Adventure and season 6-8 his eyes become green. For pajamas, he wears green footsies.
In the CGI/live-action films, Theodore has dirty blond 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 sneakers. He has dirty blonde hair and brown eyes. His pajamas consist of a matching set of a white and green striped long-sleeved shirt and pants.
Theodore and Dave have an extremely close bond. 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 special necklace for Dave in Alvin and the Chipmunks: Chipwrecked. Dave loved the necklace that Theodore made for him and Theodore loved seeing his adoptive father happy in said movie.
Theodore loves Alvin because he is his brother. However, Theodore gets upset with Alvin sometimes, though they tend to get along sometimes and work together, as in the CGI films. Theodore is usually dragged into Alvin's schemes. However, despite Theodore being sensitive, naïve and gullible, Theodore is more level-headed than Alvin.
Theodore can always count on Simon to look after him, especially in the 1980s series. Theodore stays close to Simon, because he knows Simon will protect him, because they love each other. Though at times Simon gets frustrated with Theodore sometimes because Theodore quite doesn't listen to what Simon says. Despite that, they get along fine.
Theodore and Eleanor are the most open about their relationship. They spend lots of time together and enjoy each other's company. In Alvin and the Chipmunks Meet the Wolfman Theodore gives Eleanor a necklace. They are shown to be married as adults in Big Dreams.
Theodore and Jeanette are friends, but they don't interact very often. In The Brunch Club, Jeanette stands up for him when Alvin, Simon, and Brittany tease him about his weight. In Jeanette Enchanted, he gives a witch/wizard 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 because she didn't want him to be upset when it didn't 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.