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

No I don't eat dragon, cause, uh, it's not a meal for peasants, it's a meal for kings, and I'm sort of a common man. But they don't eat us, it's a common misconception. They actually eat gold and treasure -- that's why they're always sitting on a pile of it.

Charlie

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

It's Always Sunny Quotes

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.

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