Robb Stark undertook to marry one of Lord Frey's daughters (not specifically Roslin) as a part of the pact between him and the Freys, but he breached the agreement by marrying Talisa Maegyr instead.
Lothar Frey and Black Walder arrive at Riverrun after Robb offers to renew their alliance, but Black Walder relays that Lord Frey insists that Edmure Tully must immediately marry Roslin, having "gone off" long engagements after Robb reneged on his own. Edmure initially refuses (partly because he, in contrast to Robb, is not allowed to choose between Lord Frey's daughters), 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. She glances at Robb, who turns to Walder, who gives him a rather sarcastic look. Roslin and Edmure get along quite well until Robb and Walder call for the bedding ceremony, when Roslin is carried off by Frey and Tully attendants to the bridal chamber. It is unlikely that Roslin was aware of the plans for the grislier parts of her wedding. During their wedding night, Edmure impregnates Roslin, and she later gives birth to a son - although Edmure remains a captive at the Twins.
During a conversation between Jaime and the captive Edmure, Jaime points out that Roslin has already given Edmure a son that was sired on their wedding night. Edmure then reveals that he hasn't seen Roslin since their wedding night and has never met his son.
|Game of Thrones: Season 3 appearances|
|Valar Dohaeris||Dark Wings, Dark Words||Walk of Punishment||And Now His Watch Is Ended||Kissed by Fire|
|The Climb||The Bear and the Maiden Fair||Second Sons||The Rains of Castamere||Mhysa|
In the books
In the A Song of Ice and Fire novels, Roslin is born from Lord Walder's sixth wife, Bethany Rosby. Roslin is the fifth child that Bethany bore Walder, and she has four older brothers: Perwyn, Benfrey, Willamen, and Olyvar. Members of House Rosby are known to have frail health to begin with, and Bethany's frequent pregnancies took a heavy toll on her body. Walder kept Bethany almost constantly pregnant, having a new child every year of their marriage, and ultimately Roslin's mother died not long after she was born.
Roslin is described as being the prettiest of Walder Frey's offspring. She is small with very white skin, a pretty face with a small chin, delicate nose, and big brown eyes. Between her teeth she has a small gap. Her hair is brown and reaches her waist. She has a gentle nature and a talent for music. Roslin was seventeen years old in the third novel, but in the corresponding section of Season 3 in the episode "The Climb", she is stated to be nineteen years old: this is in keeping with how most of the younger cast members have been aged-up by two years in the TV continuity.
Unlike in the TV series, Roslin is introduced to Edmure shortly after he and the Starks arrive at the Twins. Since she is so small and has narrow hips, Edmure and Catelyn are concerned about her fertility. Catelyn discusses it with Maester Brenett, and he assures her there is nothing to worry about, for Roslin's mother gave Lord Walder a child every year, five of them living past infancy.
Edmure finds it strange that Roslin is sobbing when she meets him. She tells him "I weep for joy, my lord". Only a moment before the massacre begins, Catelyn realizes the real reason for her tears: Roslin knew about the scheme and was forced to go along.
By the point the books reached, Roslin is pregnant, and has not yet given birth. According to Perwyn, Roslin has been praying that her child will be a girl, fearing (correctly) that if she gives birth to a boy, her father will have no more need of her husband.
Edmure knows about his wife's pregnancy, but is unaware that if the child will be a boy, the Freys will dispose of him. Jaime promises Edmure, among his other terms of surrender, that he will send Roslin to join him in Casterly Rock; if her child is a boy, he will serve House Lannister as a page and a squire, and when he earns his knighthood, they'll bestow some lands upon him; if Roslin gives birth to a daughter, Jaime will see her well dowered when she’s old enough to wed.