Simon Seville is the overall deuteragonist of the series and films.
He is the smartest of The Chipmunks, possessing an I.Q. just north of Einstein's. Simon possesses a very dry sense of humor, as well as a keen wit. Simon gets along great with Jeanette, who is smart, just like him. They are shy about their relationship, however it is obvious that he cares for her.
Simon is the smart chipmunk. He sometimes corrects people if they are wrong whether it's facts or proper grammar. Simon doesn't have a pronounced rivalry and developing relationship with Alvin until the 1980's series. In this series Simon and Alvin don't fight much.
Simon is the genius brother of the trio and is usually seen as the person who values the 'status quo' of action, often correcting his brothers or the Chipettes if he perceives them as wrong. He is the middle chipmunk,as revealed in the episode Grounded Chipmunk when Alvin states he has "never been alone, except those five minutes before Simon was born." Simon is a bit sarcastic but yet sweet. He has a rivalry with Alvin and they would argue because of their differences. Simon's often seen either hanging out with Theodore and sometimes Alvin or working on a new science experiment. His eyesight may not be as bad as Jeanette's because he still looks directly at the person he is speaking to and rarely bumps into things without his glasses, both of which Jeanette does. However he is sometimes a little clumsy, glasses on or not. He also has an affinity for Thomas Edison. Despite being mature, he has a meltdown in the episode Thinking Cap Trap.
Alvin exploits Simon's mastermind (less so than either series), though Simon secretly worries that they share the same gene pool. In Alvin and the Chipmunks, Simon is shown as a little less reserved and with a reduced higher intelligence at times (such as giving Dave a paperclip compass). Despite this, he still corrects people every now and again. His eyesight is so extremely poor as he's unable to see a toaster waffle fall directly in front of him without his glasses, which means that he's hyperopic or far-sighted. Simon's rivalry with Alvin isn't as bad as the 1980's and ALVINNN!!! and the Chipmunks series, but they butt heads at times.
In Alvin and the Chipmunks: Chipwrecked, Simon has an alternate personality caused by a spider bite. His alternate identity goes by the name Simone, who speaks with a French accent and frequently flirts with Jeanette. Simone is depicted to be braver and less uptight than Simon. Near the end of the movie, Simon recovers from the bite and reverts back to his usual personality.
Simon is once again the brains of the Seville bunch. He frequently corrects other people (such as in Sister Act when Alvin misspells "in" as "inn") and uses his technical abilities to help improve his standard of living (rewiring a talking teddy bear in Talking Teddy or building a robot bodyguard in Bully For You). Simon is revealed to be the eldest when Alvin indirectly mentions that he is the "middle child" (The Sub). However Simon being the eldest of the Chipmunks is confirmed by Dave in Big Bro Theodore. Simon retains his dry sense of humor while frequently replying in intellectual stammer (Clowning Around). He sometimes refuses to listen to what Alvin says by either putting his hands over his ears, rolling his eyes, or making a rude remark. Simon isn't afraid to stand up for what is right (Simon For President), though he slips on his duty sometimes (such as in To Serve And Protect) but ultimately takes the responsible action nevertheless. When it comes to opinions and experiments, Simon sometimes thinks he knows better than Dave, which leads to occasional arguments. During times Simon is stressed or upset, he can suddenly snap at others. Simon, on multiple occasions, sleep talks allowing for Alvin to take advantage of him to get his own way. Despite being mature as well as the eldest, he can show childish qualities at times, like crying for sappy moments (Summer Camp) or getting easily frightened (Monster Madness).
Simon is commonly seen wearing a dark blue top and a pair of glasses. He is the tallest chipmunk, even slightly taller than Jeanette.
Before The Alvin Show, Simon wore a red vest with an 'S' on it and didn't wear any glasses. After The Alvin Show was created, Simon was given more human features, and instead of a red vest Simon began wearing a blue sweater (yellow on the front covers of a few of the 60s Dell comics), black glasses with light gray lenses, and black shoes. The 'S' was removed for and after the show.
In Chipmunk Punk, Simon's hair is spiky as if he were a rock star. He once again wears his black glasses, blue sweater, and all black shoes to match the style of the album.
In the Alvin and the Chipmunks series, Simon wears black glasses with white lenses, a blue sweater, and a pair of white and red patterned sneakers. As the series progresses, Simon's glasses become blue as well as his eyes, though in The Chipmunks Go to the Movies episode Batmunk, playing the title character,his glasses are gold.
In the CGI/live action films, Simon has dark brown fur and blue eyes. He only wears a blue hoodie and a pair of black glasses that came from a Santa Claus toy. His appearance was modified to look more closely to a real life chipmunk.
In ALVINNN!!! and The Chipmunks, Simon has brown hair and blue eyes. He wears a two-different-shades-of-blue non-hooded dress-like sweater, dark blue jeans, white adidas sneakers with two black stripes and large, blue-rimmed glasses. His pajamas consist of a long sleeved blue T-shirt with a robot on it along with dark blue, light blue and white plaid pants.
Simon and Dave have a close relationship as Dave takes care of him along with his brothers because they are his children. Dave has confidence in leaving Simon in charge or in Simon's ability to help him with things he's not comfortable with. Sometimes Dave can get upset with Simon but not as much as Alvin.
Since the 1980’s cartoon Simon and Alvin have a rivalry, like when Simon disagrees with one of Alvin's schemes or with how far he takes them. Alvin can be jealous when Simon takes his role as a "heartthrob" with the ladies, but, nevertheless, Alvin is protective of his brother and loves him.
Simon and Theodore also have a really close relationship. Simon always seems to be with Theodore most of the time, especially in the 1983 series. Simon is also often seen to be protective of his brother. However, despite his protection of Theodore he values him just as much as Alvin and their dad Dave.
Simon and Jeanette have a very strong relationship. During the two series, their relationship can be described as best friends who have secret crushes on one another. Simon is always looking out for Jeanette and stands up for her. Simon seems very protective of Jeanette, and the two of them spend a lot a time together. In My Sister The Weirdo, it is shown that he can be jealous of others helping her instead of asking him. Particularly in Jeanette's Secret Garden, Simon becomes tired of Jeanette, she even bothers him in his sleep. In Secret Admirer, it is shown that he likes her, but he's just too embarrassed to admit it. In the 1980s series episode Big Dreams, they are married in the future.
Simon and Brittany are friends, but they don't interact much. Brittany interacts more so with Simon in ALVINNN!!! such as acting as his campaign assistant in Simon For President (helping him appeal better to the general public) and showing tough love in A Room of One's Own to get Simon to admit he misses Alvin.
Simon is frequently regarded as the oldest of The Chipmunks given his height and maturity and is often a center of controversy among fans. In the 80s series, however, Alvin is proven to be older than him in the episode Grounded Chipmunk. Simon's age in the CGI/live-action films has never been stated. Though the CGI series' websites in various countries contradict each other, some state he's older than Alvin by eight seconds while others say Simon's younger, the boys' birth order is confirmed in The Sub when Alvin refers to himself as the "middle child."
In Addicted, it's suggested that Simon is 12 or older due to the age range on the hoverboard box.
In Tables Turned, it's revealed that Simon's IQ is 154. Though this contradicts his bio for the CGI series which says his IQ is north of Albert Einstein's, who had an IQ of at least 160.
Throughout the franchise, Simon has constantly switched between wearing black glasses and blue glasses.