Carl Wheezer is a bespectacled dim-witted boy and one of Jimmy's best friends. Carl and his parents have numerous allergies, at times even to things that are incapable of triggering allergic reactions. He passionately loves llamas, and his passion for them is often used as a running gag throughout the series. He is also a bit cowardly and developed a crush on Jimmy's mom, as shown throughout a multitude of episodes. His crush on Judy becomes so extreme that he is shown an interest in exterminating Judy's husband and Jimmy's dad, Hugh Neutron. An extraterrestrial character named Doppy, with similarities to Carl, is a main character in Planet Sheen.
His extreme obsession for the llama originated when his parents expanded their cable lineup and he became devoted to a show called Llama Boy The Super Hero. Since his obsession began he has gone to great lengths to express his love for them, going as far as to join a fan club called The Llama Love Society, which he likely created himself.
It is worth noting, Carl is afraid of lima beans, germs, and scary stories. He is also known to have asthma, and is very stressed about his multitude of allergies and is terrified of getting any sort of illness. Also, when he is tired, his mom makes him warm milk, rubs his tummy, and sings "nani nani nani".
He frequently shows off his vocal abilities performing improvised and poetically expressed rhymes.
"Carl is Jimmy's best friend, but they couldn't be less alike. Carl is cautious and, well, kind of chicken, and he's prone to wheezing and carsickness. He demonstrates his asthma inhaler for show and tells, and at the amusement park, Carl spends his day trying to beat the guy who guesses allergies. But through it all, Carl is Jimmy's loyal buddy and biggest fan...and the crash-test dummy for Jimmy's arsenal of inventions."
Carl has peach skin with freckles. His large, almost pear-shaped head characterizes him with small black eyes that are reasonably close together. He has short, thick red hair that forms clump-like structures of varying sizes at the top of his head and the eyebrows of the same color/texture.
His body and face show that he is overweight, as they are round and don't reveal much about skeletal structure, though his arms and legs don't seem to have stored so much fat. He wears large brown circular glasses and has a short round, yet broad nose. He wears a light orange and coral short-sleeved vertically striped shirt, under a pair of orange suspenders, which connect to his dark green trousers. He also wears a pair of what appears to be light brown loafers.
In the series, Carl‘s suspenders on his pants are nowhere to be seen, unlike the movie.
Carl is quite greedy, loving to devour not only food but objects such as toys and newspapers. When his parents expanded their cable line-up, Carl found himself focusing on a channel devoted to llamas and a show about a superhero called Llama Boy, the Super Hero. He has adored llamas ever since, going so far as to join a fan club, the Llama Love Society (which he likely created, since he has shown to be the only member). He even turned into a llama once. He is allergic to almost everything and has asthma. Carl often unknowingly gives Jimmy his brilliant ideas.
It has revealed in "The Incredible Shrinking Town" that he took ventriloquist lessons and can virtually imitate and sound like everyone, a feat nobody else in Retroville can achieve. It is also a hint that he has some sort of dislike for Sheen, even though he's one of his best friends. Carl enjoys playing a video game called Llamapalooza. It is also revealed in I Dream of Jimmy and Carl Wheezer, Boy Genius that Carl is actually self-conscious and insecure about his unintelligent, and envious of Jimmy and wants to steal his life. Carl seems to specifically envy Jimmy's IQ and all the attention he gets from girls and is always upset that Jimmy makes him test his inventions since he has sometimes harmed.
- "What???!!!" - Jimmy for President
- "OK, I'll have one more and then that'll be it." - Jimmy Neutron: Boy Genius
- "Um, yes, are you going to finish that croissant?" - Lights! Camera! Danger!
- "Time to take out the trash!" - Sleepless in Retroville, Attack of the Twonkies
- "OUCH! Sheen, that hurt!" -Broadcast Blues
- "Ow, my scapula!" - Carl's catchphrase
- "STOP IT, STOP IT! CAN'T YOU SEE YOUR CONSTANT FIGHTING IS TEARING US ALL APART?!" - Raise the Oozy Scab
- "Llama, llama, llama!" -Jimmy Neutron: Boy Genius
- "I don't know, Jimmy..." (Usually when testing one of Jimmy's inventions)
- "Saying goodbye to your kids is always painful, Jimmy." - Who's Your Mommy
- "It's a good thing my underwear was clean." -King of Mars
- "Guys, can I have my pants back?" -King of Mars
- "But why did you wink? Why'd you gimme a dollar?! My brain hurts!!!" -Jimmy for President
- "Jimmy Neutron, he's a spy! He's a kid and his hair is really high! He's got a friend named Carl who is really cool. And a friend named Sheen ... WHO WAS HELD BACK FROM SCHOO-OOL!" -Operation: Rescue Jet Fusion
- "Thanks. I saw that in a soap opera once..." -Raise the Oozy Scab
- "Who's eating whom??" -Stranded
- "Flying metal chicken..." -The Eggpire Strikes Back
- "I want my mommy!!!! ...And some fudge." -Hall Monster
- "So?? Pinching still hurts!" -Normal Boy
- "Thanks, guys! I thought I was gonna be snake food!" -Beach Party Mummy
- "Aw, come on, guys. Eighth place isn't so bad..." -Battle of the Band
- "RUBY MADNESS! RUBY MADNESS!" -A Beautiful Mine
- "Jimmy, why is it so unseasonably humid?" -Sorry, Wrong Era
- "Will you be my mommy?" -Operation: Rescue Jet Fusion
- "I'm a bad Watson!"-Crime Sheen Investigation
- "Uh, I want sour. No! Uh, salty. No! I don't know which one to choose!" - Krunch Time
- "HELLO, RETROVILLE! I hope you had a scrumpdillyicious day! Let's wish a big old happy birthday to old lady Nusbaum who turns 102 today! Here's a nice big weather center hug from Dr. Carl!" - Lady Sings the News
- "Do I have something on my lip?" -Who's Your Mommy
- "Who likes fruit chewies?" -The Great Egg Heist
- "Uh, Jimmy! I know you're kinda busy, but, uh... HELP!!!" -When Pants Attack
- "Somebody do something!" -The League of Villains
- "Hey, Jimmy, wanna see a frog?" -Jimmy Neutron: Boy Genius
- "But Jimmy, I'm allergic to infrared!" -The Great Egg Heist
- "I don't wanna be a bubble boy!" -Journey to the Center of Carl
- "Uh, Mrs. Neutron? May I use your restroom because I really gotta...never mind" - Runaway Rocketboy!
- "Yeah but his heads still gonna explode! And Sheen's still our best friend somewhere deep down." -Sheen's Brain
- "RED PIRATE!!!" -Monster Hunt
- "We need more crab grass, MORE!" -The Feud
- "Baby Llamas in the meadow, baby Llamas in the meadow. Oh no, one of them escaping, he's heading towards town, town rhymes with crown! Jimmy and I disguised the Brain Drain Helmet as a crown!" -Sheen's Brain
- "Hey, you guys taste pretty good." -Journey to the Center of Carl
- "Hey, this astronaut food isn't too bad." -Jimmy Neutron: Boy Genius
- "This is the end. My entire life's flashing before my eyes, good bye world!" -Sheen's Brain
- "Oh, no! I'm in my underwear at the maaaaaallllll!!!!!" -I Dream of Jimmy
- "You're the one who fell!" -The League of Villains
- "Oh, no... Goddard's eaten Jimmy!" -Ultra Sheen
- "Let's go eat." -Professor Calamitous, I Presume
- "Oh yeah, I'm bad! I defeated Ultra Lord!" -Ultra Sheen
- "Fix us Jimmy! I don't wanna be this old for fifty more years!" -Grumpy Young Men
- "MUMMIES!!!" -Beach Party Mummy
- "The Yolkians gave me a sipping chicken." -The Eggpire Strikes Back
- "Hop the, oh, I'm sorry Jimmy, I can't. You know some people can't whistle, some people can't snap their fingers, well I can't hop!" -The Phantom of Retroland
- "I don't sniff my feet! It just looks that way when I'm doing yoga!" -Lady Sings the News
- "He said I was gonna be dust in the wind!" -Send in the Clones
- "Oh, nothing. I think it's great that you delivered all the presents. And now everyone will realize that there really, is no such thing as SAAAAANTA!!!!" -Holly Jolly Jimmy
- "Ah, Llama Boy is how you say old school. May I say, you look enchanting tonight." -Nightmare in Retroville
- "What happened to the Mark III?" -Jimmy Neutron's Nicktoon Blast
- "Hi Jimmy, Can I touch your uniform" -Hall Monster
- "You should give it another chance" -One of Us
- "Get back, Ultra Lord! Don't make me pinch you!" -Normal Boy
- (In flashback) "I'm okay!" -Is This Cute?
- Carl is similar to Filburt from Rocko's Modern Life.
- In the pilot, shorts, movie, and early episodes, Carl's voice was deeper, but later on the series, his voice became more high pitched.
- In Win, Lose and Kaboom! Carl mentions that he goes to Bolbi's house for cookies, while he was voting him off.
- Carl makes a cameo in a flashback in the Planet Sheen episode, Is This Cute? when Sheen saw Doppy and saw the strong resemblance to Carl, but then disregards if after the flashback.
- Carl was going to be part of the series, but Nickelodeon executives decided to make it only about Sheen and Doppy was created.
- Carl 'hosted' Rob Paulsen's "Talkin' Toons Podcast" on July 6th, 2012. In it, he reveals many hidden details including the fact that Doppy was just him in a costume, Sheen is currently in juvie (back on Earth after returning from Zeenu), and at one point he had a crush on Cindy. This most likely might be non-canon, since none of it has been confirmed in animation or any other form of media.
- It revealed in The Incredible Shrinking Town when Carl took ventriloquist lessons. He was going to sound like Cindy, but never got the chance. However; in the episode, he did sound like Sasha and Zix.
- In Journey to the Center of Carl, It reveals that Carl eats his toys because it tastes fruity and chewy.
- When Carl's suspenders had removed in the TV series, the parts where his suspenders pulled up his pants were still in the state of being pulled up when they had removed. This was fixed in Jimmy Neutron's Nicktoon Blast, and the episode Make Room for Daddy-O but was still present in some scenes in the second season episodes Sorry, Wrong Era, Grumpy Young Men, Operation: Rescue Jet Fusion, Return of the Nanobots, Holly Jolly Jimmy, and Love Potion 976/J and his cameo in the Planet Sheen episode Is This Cute?
- Carl has asthma and has an inhaler because of it, although the inhaler appeared less as the TV Series progressed.
- He, Jimmy, and Sheen are the only main characters to appear in every episode.
- He appears in the mobile game Nickelodeon Pixel Town (only available in Singapore, Malaysia, and the Philippines). You can get him if you merge Wanda and Tooth Fairy from The Fairly OddParents in the memory center.
- His phone number is 555-2275. This can be seen on Jimmy's watch.
- Carl has spawned numerous memes, one of them being the "Are you going to finish that croissant?", where it sounded like he said "Quasol".
Minor and One-Shot Characters: Ultra Lord | April the Gorlock | Captain Betty | Commander Baker | Corky Shimatzu | General Abercrombie | Flippy | Jet Fusion | Amanda Neutron | Kari Neutron | Anabelle Neutron | Gomer Neutron | Granny Neutron | Newt Neutron | VOX | Ebenezer Wheezer | Martha Wheezer |