She was previously the intended bride for Robb Stark.
Roslin is the 19-year-old daughter of Lord Walder Frey, Lord of the Crossing. Unlike the majority of Lord Walder's brood, Roslin is quite beautiful. She was intended to marry King Robb Stark as part of an alliance between her father and him (Walder had given Robb a choice of any one of his many daughters or granddaughters, but Roslin would have been the obvious choice), but Robb fell in love with and married Talisa Maegyr instead.
Lothar Frey and Black Walder Rivers arrive at Riverrun after Robb Stark offers to renew their alliance, but Black Walder relays that Lord Walder insists that Edmure Tully must immediately marry Roslin, having "gone off" long engagements after Robb reneged on his engagement. Edmure initially refuses, but eventually consents after bowing to family pressure as well as his desire to make amends for his blunder at the Battle of Stone Mill
Edmure does not meet Roslin until she is unveiled at the wedding. Having seen Walder's other daughters, Edmure was shocked and relieved to see Roslin at last. The newlyweds got along quite well until Robb and Walder called for the bedding ceremony, when Roslin was carried off by Frey and Tully attendants to the bridal chamber. It is not known what, if any, involvement Roslin had in the planning or execution of the grislier parts of her wedding. Her status after the event is uncertain, although Edmure remains a captive at the Twins.
The fairest of the Frey girls and newly wed to Edmure Tully, Roslin was married off as part of a bargain her father made with Robb Stark. Her wedding night erupted into a violent battle that obliterated the Stark army and became known as the Red Wedding.