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When Dr. Doofenshmirtz asked Vanessa to pick up blueprints at Blueprint Heaven for a Space Laser-inator Ray, Carl is dressed as the lady at Blueprint Heaven. He accidentally gives real plans for a Space Laser-inator Ray to Ferb and gave Vanessa the blueprints for an Ice Cream Machine. Major Monogram then calls him a rookie for his mistake ("I Scream, You Scream").
When Major Monogram is unable to debrief Agent P due to a back injury, Carl does takeovers for him instead. He made the close-up too close to his face, making his glasses the only thing visible ("The Flying Fishmonger").
When Dr. Doofenshmirtz built the Freeze-inator, it was Carl who debriefed the mission instead of Major Monogram due to the Major being unable to move because he was shot by the Freeze-inator. He accidentally knocks Major Monogram over, thus showing that he is very clumsy ("It's About Time!").
Carl also has his own theme called Carl the Intern. He once had to disguise himself as an ice cream man because Baljeet and Buford were watching Perry and Perry couldn't get his mission. So when Baljeet and Buford were ordering Carl's ice cream, Perry went through an escape door in the ice cream truck. Then Carl zooms off, leaving Baljeet clueless about where Perry went ("Swiss Family Phineas").
During the Father-Daughter picnic, he built a robotic replica of the Flynn-Fletcher family to replace Lawrence Fletcher for the Father-Daughter picnic competition while he was restoring his memory. He also helped make the switch between the robotic version and the real Mr. Fletcher and congratulates Agent P until a kid sees them, forcing him and Agent P to quickly leaves the scene ("I Was a Middle Aged Robot").
When Phineas and Ferb create a device to reverse the effects of gravity, Carl goes undercover as a boy, thinking that Phineas and Ferb are in league with Doofenshmirtz. Since Agent P is close to the boys, Major Monogram has him find Agent G, giving him suspect locations, some being the moon and Monte Carlo. Carl discovers that the boys are just having fun and returns with the news ("Undercover Carl").
When Dr. Doofenshmirtz abducts Major Monogram, Carl gives Perry his briefing, describing Major Monogram going to Tokyo for the international Good Guy convention. He also shows the letter made from cut-out magazine letters and mentions Dr. Doofenshmirtz going to Tokyo with his daughter. However, he passes the whole situation off as a coincidence and gives Perry the day off ("Phineas and Ferb Summer Belongs to You!").
While attending Squirrel-Con, Carl finds Agent P's hat, and since Major Monogram is too busy to do anything, Carl goes to find out what has happened to Agent P and gets captured by Doofenshmirtz ("The Remains of the Platypus").
By accident, he fell under the influence of Doctor Doofenshmirtz's Ultimate Evil-inator, using his status within the OWCA to launch a full-scale attack on the Tri-State Area, momentarily slowed down by Perry's interference before he strangely disappears ("Where's Perry?").
His plans to conquer the Tri-State Area were severely damaged as Perry locked the control panel and only his paw print could unlock it. To this extent, Carl goes on a hunt for the lost platypus and winds up in Africa, cornering Agent P in the unexplored gorge. However, Agent P does not give into evil Carl and engages his robot duplicates in a full-scale battle with aid from the African wildlife, a confused Candace, Major Monogram and Doofenshmirtz before Doof uses his newly built -inator to turn Carl back to his normal self. He then returns home with Monogram and Doof, but not before being promoted to the "title" of "paid intern" ("Where's Perry?").
He would eventually make Commander in O.W.C.A., and took over Monogram's position when he went to a better place.
Carl is a bit careless and sometimes forgets to focus the camera, causing Major Monogram to be humiliated during some occasions. In one occasion, in particular, Carl accidentally didn't zoom in, revealing that Major Monogram isn't wearing any pants. He is also desperate for a girlfriend. When Agent P went into his lair while his brain was in Candace Flynn's body, they at first thought there had been a breach in security and that it was a real girl, causing Carl to get very excited. They were soon under the impression that it was merely a disguised Agent P when his fedora fell onto his head. Carl was very disappointed, he walked off-screen dejectedly, saying, "Man! I thought it was a real girl" ("Does This Duckbill Make Me Look Fat?").
Carl has red hair and wears purple framed glasses, dark green eyes and he also has freckles. His outfit consists of a white shirt with three white buttons, light brown pants, and orange sneakers. He also used to wear braces. ("Phineas and Ferb The Movie: Across the 2nd Dimension")
When under the influence of Doofenshmirtz's Ultimate Evil-inator, he was seen wearing a black leather jacket, a pair of shades over his eyes and jeans. However, he was mostly seen in his normal clothes despite his new "evil" persona.
Major Monogram calls him a rookie. This might mean that he might have started working there at the beginning of summer ("I Scream, You Scream"). This belief is contradicted in a flashback as it shows Carl secretly assigning Perry to the Flynn-Fletcher family 5 years in the past. However, it could just mean that Carl has been working for Monogram for about 5 years as some sort of volunteer and has only been promoted to 'unpaid intern' that summer. Monogram said "unpaid intern in no time" so he should have been promoted at least on his 3rd or 4th year working for the O.W.C.A. ("Phineas and Ferb The Movie: Across the 2nd Dimension").
Carl's age is not certain, but he is most likely a sophomore going to be a junior next school year, as he said he "based his sophomore thesis on quakes and seismic events" ("At the Car Wash").
He knows how to cook spaghetti, but his forgetfulness prevents him from cooking it well ("At the Car Wash").
Carl knows how to work a camera, clear a paper jam, format, spell check and text edit ("Swiss Family Phineas").
He is now officially a paid intern, though the OWCA won't be paying him, it's just a title. ("Where's Perry?')
So far, Carl is the only one in the O.W.C.A. (other than Perry) to know of Vanessa and Monty's relationship. Apparently, Monty owes Carl because he didn't tell Major Monogram about his secret relationship with Vanessa ("My Sweet Ride").
He's on Major Monogram's heebie jeebie scale. ("Fly On The Wall")