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.

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

It's Always Sunny Quotes

Dee: You've been really stressed so I thought I'd take you to a spa day. Just the two of us.
Charlie: A what day?
Dee: Spa day
Charlie: What is this word, spa? I feel like you're starting to a say a word and you're not finishing it. Spaghetti? Are you taking me to a spaghetti day?

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