Charlie Kelly is one of the co-owners of Paddy's and is a childhood friend of Mac and Dennis. Charlie is definitely not the smartest of the group and is constantly is financial trouble, lives in a crap apartment, and is stuck doing "Charlie work" (aka janitor stuff). Frank ends up living with Charlie and there's a pretty good shot he's actual his father after a one night stand with Charlie's mother.
Mac: Fight Milk! The first alcoholic dairy based protein drink for bodyguards!
Charlie: By bodyguards! I drink it every morning so I can fight like a crow.
Principal: I'm a little confused, are you telling me this photo of Bruce Jenner is your resume?
Charlie: Well, when I showed up this morning I didn't have a formal resume on me so i was sort of hoping the photograph of Mr. Jenner could represent the standard of excellence I'm hoping to bring to his position.
Principal: And you're looking for a job as a substitute teacher?
Charlie: Substitute janitor.