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Theodore "Theo" Seville is the main hero in Alvin and the Chipmunks anime series and also anti hero in Kim Possible series, Phineas and Ferb series, Teen Titans series, and Teenage Mutant Ninja Turtles series. He is the youngest of the group and is best described as shy, loving, sensitive, gullible, trusting, naive and innocent. He is the younger brother of Alvin and Simon, the youngest son of Vinny, and also the youngest adoptive son of Dave Seville. He wears a green sweater as his iconic color. In Alvin and the Chipmunks: Trouble in Rio de Janeiro, when Alvin and Simon turned him into a horse, he wears a green feather headdress that replacing his green Sherlock Holmes' hat and green saddle on his back.
In Alvin and the Chipmunks: House of Flying Butcher Knives, he was the only one who is suspicious about the Purple Dragons' intentions and was shocked after he finds out that they were secretly working for the Foot Clan all along. Later, he learns of an assassination plot against they adoptive father Dave "David" Seville (Jason Lee) and does his best to warn the Belgian spies. When his adoptive father was murdered, he works with an orphaned female ninja named Karai (Kelly Hu) in order to seize power from the Purple Dragons.
He and the Purple Dragon gangs were going to commit robberies, and it was an unhappy one as Tsoi (James Sie), Fong (Andrew Kishino), Hun (also played by Andrew Kishino), and Sid (Andrew Kishino) abused him like a helpless puppy and drank constantly as well as blamed him for everything that upset Hun, Fong, Sid, and Tsoi when things go wrong; but they knew that betrayal was not an option and it was seen as a sin during their time where it can put them in a very bad situation in their society and Theodore, along with his two older brothers Alvin and Simon, would be separated from their adoptive father, if that happened.
While he was terrified of their rampages and when they hit him with a baseball bats, Theodore was brave and fearless boy, despite of his pacifist behavior and his fear was probably an act as he was seen in other scenes talking to Karai and the others on helping Dave Seville. When Dave Seville was murdered, he knew Hun, Tsoi, Fong, and Sid had something to do with it and that pushed him to take the Seville's house from Hun, Tsoi, Fong, and Sid when they became a a cold-blooded serial killers by treating the people of Los Angeles without respects in order to save the Southern California by destroying the four turtles as well as avenging their dear adoptive father for his death and avenge himself for the mistreatment Hun, Fong, Tsoi, and Sid put his throughout their entire friendships.
During their trials, they angrily yells at him, to which Theodore replied sternly that if they were a better persons and kinder mans he wouldn't have the determination to be a brave fighter as their cruel actions towards him reminded him what he had to stop or else all Southern California would suffer from their tyrannies like how Hun, Tsoi, Fong, and Sid to him and how they are the reason his adoptive father is dead.
In order to get his revenge against Hun, Tsoi, Fong, and Sid for conspiring against Dave Seville and being part of his demise, even thought they claims they didn't knows, Theodore sentenced the four of them to deaths by hangings. As Hun, Tsoi, Fong, and Sid screams and begged them to stop, Theodore was remorseful for a moment, but turned cold again as he remembered what Hun, Fong, Sid, and Tsoi did to him and how they are the reason his adoptive father is dead.
As Theodore was getting ready for his trip to Tucson, Arizona with his two older brothers Alvin and Simon, and their friends Brittany, Jeanette, and Eleanor, Karai, Leonardo, Michelangelo, Donatello, and Raphael come in to bid him farewell as they were heading to Saint Augustine, Florida and are unable to attend Theodore's middle school graduation party. Before heading to Tucson, he senses Karai's guilt and tells her that she was not to blame Dave Seville and that nobody could predict the tragedy, and that bringing judgement upon their abusive teammates and changing the laws to do it was the only way to keep Southern California from chaos and turmoil will be maintained and he will bare the sin of the deaths of Hun, Tsoi, Fong, and Sid and protect Southern California till the day he dies. In the end, Theodore eventually feels sympathy on Karai for the death of her biological mother Tang Shen in a burning Japanese restaurant back when she was an infant. In Alvin and the Chipmunks: Chasing Camille Leon, Theodore, along with Rufus (Nancy Cartwright), Jeanette, and Eleanor are transformed into a horses by Kim Possible (Christy Carlson Romano). They pull the tomato coach carrying Alvin, Simon, and Brittany to the Seville's House so that they can have a chance to pursues Camille Leon (Ashley Tisdale), hoping to prevent her from escaping with the ransom Japanese yens. When the spell breaks at dawn, Theodore, along with Jeanette, Eleanor, and Rufus are turned back to their normal selves.
- (during the mirror match scene) Impersonating me again Demyx?
- (during the mirror match scene) Your charade ends right now!
- Younger brother of Alvin and Simon
- Youngest son of Vinny
- Youngest adoptive son of Dave Seville
- Boyfriend of Eleanor Miller
- Protected by Donald Walls from getting killed by Hun (Eric Bauza) and his men in Alvin and the Chipmunks: Chinese American Assassins.