Deandra "Sweet Dee" Reynolds is Dennis' twin sister and the main bartender at Paddy's.  Sweet Dee was definitely considered a loser in high school thanks in part to her back brace she had to wear back then.  Sweet Dee failed out of U Penn, where she studied psychology.  Now, she's mostly a failed actress and stuck as a bartender with her brother and his friends.

Deandra "Sweet Dee" Reynolds Quotes

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

Dee: Oh, you are being ridiculous. He's a professional football player.
Mac: No, look, I'm not talking about killing the guy. I'm just talking about going up there with a group of dudes and intimidate him, maybe break his arm.
Dee: You can't break Tom Brady's arm.
Mac: Oh yes, I can! No more Super Bowls for that pretty boy.

It's Always Sunny Quotes

Dennis: I am having feelings again. Like some kind of fourteen year old kid. You remember, feelings right?
Mac: Yeah. I have feelings every single day of my life.
Dennis: Do you?
Mac: Are you saying you don't have feelings?
Dennis: What I'm saying is a built up a shell.. a shell around myself. A cold, calculated shell that couldn't be broken by anything but marriage.

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