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: I'll totally pull a Good Will Hunting on those kids and that'll put them in their place.
Mac: How you gonna do that?
Charlie: Well, you've seen the movie right?
Charlie: So all I gotta do is, I'll ask them some big shot, like math or science, history-type college question aand that will totally stump them by knowing a lot more about the answer than they do.
Mac: In that movie, Matt Damon played a genius janitor, you're just a janitor.
Charlie: Right, you stumped me with that one.
Mac: He doesn't have any poison.
Charlie: I don't have any on me, but I do keep some in my fridge at home in the relish jar.
Frank: There's poison in that jar? I thought I was allergic to pickles. What's in the jar with the skull and crossbones?
Charlie: Well that's mayonnaise. It's a decoy.
Frank: And the mayo?
Charlie: That's shampoo.
Frank: You're telling I've been putting shampoo on my sandwiches?
Charlie: If you've been using the mayonnaise, then yeah, probably.