Carl likes to eat, and this caused him to be overweight. 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 as he is shown to be the first 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 is revealed in "The Incredible Shrinking Town" that he took ventriloquist lessons, and that he can virtually imitate and sound just like everyone. It is also a hinted 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 he is actually self-conscious about his unintelligence, 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 is sometimes harmed.
- In I Dream of Jimmy, he secretly dreams of Jimmy being his servant and tortures and mocks him in front of their class. In his dream, he is a genius and Jimmy is unintelligent, so Carl makes him look like a fool in front of the class and forces him to test inventions.
- In Stranded, he secretly hoards food while his friends are starving. Libby and Sheen become justifiably angry with him and try to throw him overboard, after they see if he has more food on him.
- Carl became the main antagonist in Carl Wheezer, Boy Genius. He betrays Jimmy for a chance with a girl he's never met, and starts verbally abusing him, even after the many times Jimmy saves his life. When Jimmy saves Carl for the third time, Carl blames Jimmy for everything (even though he was highly responsible) and pins all his insecurities on Jimmy. Jimmy punches Carl in the face and decides to tell Elke the truth. At the end, Carl shows no remorse and doesn't even apologize to Jimmy.
- Sometimes, Carl stalks Judy to the point where he apparently wants to do away with Hugh.
Carl has peach skin and many freckles. He has small black eyes and red hair. He often wears an orange-and-red striped shirt, a pair of green pants (shown with suspenders in the movie and shorts), a pair of brown loafers, and glasses. Carl has also been left in his underwear on numerous occasions. Carl is shown to wear briefs. In the pilot, his glasses were bigger, he wore a plain shirt and overalls, had regular eyes and had a different hairstyle and a larger nose.
- "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
- "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, c'mon, 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 infared!" -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
- "A 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 thems escaping, he's heading towards town, town rhimes with crown! Jimmy and I disgused the Brain Drain Helment 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 fifthy 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
- He has a crush on Judy, which was only noticeable after the effects of Jimmy's love potion in Love Potion 976/J.
- In the pilot, shorts, movie and early episodes, his voice was more deep and melancholy. Now, his voice is more high-pitched and whiny.
- It was stated by himself that he as always wanted to touch a llama in the movie, which he does in a later scene.
- Sometimes, Carl soils himself.
- Jimmy, Carl and Sheen are the only characters to be in every episode of the original series.
- Out of all the the character design changes, Carl's one is the biggest.
- Carl can't read with his glasses on.
- 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 it after the flashback. Carl was going to be part of the series, but a Nickelodeon executive decided to make it just about Sheen and Doppy was created.
- In Win Lose and Kaboom! we learn that Carl goes to Bolbi's house for cookies.
- 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 not be canon, since none of it has been confirmed in animation or any other form of media.
- It was revealed in The Incredible Shrinking Town, that 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 (Cindy's mom) 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 were removed in the TV series, the parts of where his suspenders pulled up his pants were still in the state of being pulled up when they were removed. This was fixed around season two, but was still present in one scene with Carl in Love Potion 976/J and his cameo in Is This Cute?