Ryman is one of the Freys sent by Walder Frey to join Robb Stark's cause. When his father dies from a wound received at the battle at Oxcross, Ryman becomes the new heir to the Crossing. After Robb Stark marries Jeyne Westerling and thus breaks the deal he had with the Freys, Ryman leaves Robb's camp with the Frey men and returns to the Twins. Robb later claims that he might have been able to make amends with the Freys, had Stevron still been alive but that Ryman was far to stubborn to talk to, with Black Walder going as far as threatening Robb's wife life. Ryman participates in the Red Wedding, before the massacre Catelyn notices he is sweating excessively. He kills Dacey Mormont with an axe. Merrett Frey claims Ryman had a large part in planning the Red Wedding.
He then leads 2000 Frey men to assist the siege of Riverrun, which the Iron Throne has granted to Walder Frey's second son Emmon Frey, who is married to Tywin Lannister's sister. It is currently being held by Edmure Tully's uncle Ser Brynden Tully, "the Blackfish". During the siege he has gallows built right before Riverrun and parades the captive Edmure, claiming that he would hang Edmure should Riverrun not surrender. The Blackfish however doesn't surrender and Ryman has Edmure brought back to camp, only to put up the same facade the next day. When he rides to the gates to parley, an arrow is shot at his horse causing him to fall off. When Jaime Lannister arrives to lift the siege, he takes a disliking to Ryman's method as they make him seem weak and sends him back to the Twins, giving command of Ryman's force to Edwyn Frey. Ryman gives Robb Stark's crown to a whore, but Jaime tells him to leave the crown. On the way, Ryman and his troup of three knights and a dozen men-at-arms are attacked by outlaws, presumably the gang of Lady Stoneheart, and are all hanged. Robb's crown ends up in Lady Stoneheart's possession, implying Ryman took it with him and it was Stoneheart who killed him. When news of it reaches the siege camp, Jaime is bothered by the audacity of the outlaws who kill the heir to the Crossing not far from the Twins. Ryman's son Edwyn suspects that Black Walder is somehow involved in his father's death to become heir of the Twins, even though Ryman's bastard uncle Walder Rivers says he has no proof.