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.

Dennis: It's because of those bison fingers.
Dee: I do not have bison fingers!
Frank: Yeah you do. Your mother had to have a c-section because of those hands.

It's Always Sunny Quotes

With real power comes real responsibility and I don't want any of that shit. I just want the money and the illusion of power. And puss.

Dennis

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.