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

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.

Charlie: We'd be two cool, straight dudes married together.
Frank: Oooh. Well, I never thought of it that way. Two dudes getting married, that doesn't seem very gay.

It's Always Sunny Quotes

I gotta get my pump on.

Mac

Dee: Who slams a door?
Frank: Babies.
Dennis: That guy has some real growing up to do. Have some repect for Christ's sake... I am legend.