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.

Charlie Kelly Quotes

Frank: A cowboy and a teacher walk into a store..
Charlie: Well that doesn’t make any sense. Cowboys hate teachers, they’re independent thinkers!

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.

It's Always Sunny Quotes

I was a five star man before the internet and I’m a five star man now. I just gotta shed the dead weight. God damn it!

Dennis

Naked pics online? That's disgusting. On a website? There's so many of them though. Where? Which one?

Mac