Carl is an unpaid intern at the Animal Agents Agency. He basically does all the secretary work but sometimes, if all the agents are busy, he steps in for some secret missions at which point he then plays the "Carl" theme song. Carl is just 19 and is still in college at the same time.
Carl was created along with the AAA and has been there ever since. There's not really much else to say.
Carl grew up as a bit of a nerd in school. His father was Mr. LeaderoftheAnimalAgentsAgency (yes, all in one). Carl wanted to be a secret agent but his dad said he wasn't fit for it. When his father mysteriously disappeared one day (no, Carl didn't do it; he's a nice guy), Mr. Hazmat took over and hired him. However, Carl is still an unpaid intern secretary, but he believes he will be a secret agent one day.