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.
Dennis: We also have the bird with teeth.
Charlie: Wow, okay. And I'm assuming the teeth are fake, yes?
Dennis: Yes. Well they're not really human teeth if that's what you're asking
Charlie: No, I mean did you discover a bird with teeth in this fashion?
Dennis: That does not exist in nature.
Charlie: Okay, I'm not sure, but either way, good glue work.
Charlie: We'd be two cool, straight dudes married together.
Frank: Oooh. Well, I never thought of it that way. Two dudes getting married, that doesn't seem very gay.