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

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.

Your head of security just choked himself out.

Charlie

It's Always Sunny Quotes

With real power comes real responsibility and I don't want any of that shit. I just want the money and the illusion of power. And puss.

Dennis

Dennis: I am having feelings again. Like some kind of fourteen year old kid. You remember, feelings right?
Mac: Yeah. I have feelings every single day of my life.
Dennis: Do you?
Mac: Are you saying you don't have feelings?
Dennis: What I'm saying is a built up a shell.. a shell around myself. A cold, calculated shell that couldn't be broken by anything but marriage.