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.

Mac: Fight Milk! The first alcoholic dairy based protein drink for bodyguards!
Charlie: By bodyguards! I drink it every morning so I can fight like a crow.

It's Always Sunny Quotes

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

Mac: Fight Milk! The first alcoholic dairy based protein drink for bodyguards!
Charlie: By bodyguards! I drink it every morning so I can fight like a crow.