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

Well it's a cock ring, for, uh, all my lovers.

Dee

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

I was a five star man before the internet and I’m a five star man now. I just gotta shed the dead weight. God damn it!

Dennis

Naked pics online? That's disgusting. On a website? There's so many of them though. Where? Which one?

Mac