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.
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.
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?