Robb was suppose to marry one of the Frey girls (the ones from the water crossing) and in the books marries some chick from a lesser house cause he bangs her and want's to protect her honor...or something.
The one in the TV show is suppose to be more of a full character cause boobs. and they wanted to paint Robb and her relationship in a slightly different light to make for good TV drama.
Cause this.