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

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.

Dee: Who slams a door?
Frank: Babies.
Dennis: That guy has some real growing up to do. Have some repect for Christ's sake... I am legend.