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

Charlie: what is your spaghetti policy?
Dee: Are you hearing this? He doesn't belong in a place like this.

Dennis: We also have the bird with teeth.
Charlie: Wow, okay. And I'm assuming the teeth are fake, yes?
Dennis: Yes. Well they're not really human teeth if that's what you're asking
Charlie: No, I mean did you discover a bird with teeth in this fashion?
Dennis: That does not exist in nature.
Charlie: Okay, I'm not sure, but either way, good glue work.

It's Always Sunny Quotes

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.

Let's pull up our bootstraps, oil up a couple asses, and do a little plowing of our own. POW! (gestures a fist punch up an invisible ass)... Not gay sex.

Mac