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Edwyn Frey is a great-grandson of Walder Frey. He is the eldest child of Ser Ryman Frey, who is the eldest child of Ser Stevron Frey, who was Walder Frey's eldest child. Edwyn is married to Janyce Hunter and has one child, Walda. He has a feud with Ryman's second son, Black Walder Frey, who desires to succeed to the Twins himself. They have another brother, Petyr "Pimple" Frey. Black Walder is believed to be having affairs with many of the women round the Twins, including the wives of both his brothers, casting Walda's parentage in doubt.
He is described as being a pale slender man with a pinched nose and lank dark hair. He has a cold attitude. Ser Daven Lannister describes him as being "Not as thick as his father, but as full of hate as a boil's full of pus." However Edwyn is described as being not a man of the sword.
Walder Frey makes a deal with Robb Stark to agree to marry a Frey in exchange for his support in the War of the Five Kings. In the Battle of Oxcross Ser Stevron Frey is wounded and dies a few days later, making Ryman Frey heir to the Twins.
After Robb takes Jeyne Westerling's virginity he marries her to preserve her honour. The Freys leave his campaign in anger. Later they tell Robb they'll join him again if his uncle Edmure Tully, Lord of Riverrun, marries Roslin Frey, a half-great-aunt of Edwyn. When Robb arrives at the Twins, he is greeted by Ryman and their sons. Edwyn acts politely and tells Robb his host can take shelter in three tents.
Edmure marries Walder Frey's daughter. When Dacey Mormont asks Edwyn for a dance, he wrenches himself away. Catelyn has begun realizing something is wrong, especially when the musicians start playing The Rains of Castamere. As Edwyn hurries for the door she grabs his arm and feels armour. She slaps him hard enough to break his lip, but he shoves her aside. When Robb moves to confront Edwyn, he is hit by quarrels from the crossbowmen in the gallery. The wedding was a trap, the Freys and Robb's treacherous bannerman Roose Bolton murder Robb, Catelyn and most of their troops, taking Edmure and other nobles hostage. The three tents were rigged to collapse and are set alight. This becomes known as the Red Wedding.
Later Edwyn's brother Petyr Pimple is captured by the Brotherhood without Banners. One of his half-great-uncles, Merrett Frey, goes to Oldstones to deliver the ransom. Merrett thinks of Ryman as foolish and of his sons Edwyn and Black Walder as worse, as both intend to throw most of their relatives out of the Twins. Merrett hopes to win the favor of the main Frey branch by ransoming Petyr. However he finds Petyr has already been hanged. Lady Stoneheart, the resurrected Catelyn Tully, reveals herself and at her command Merrett is hanged.
For their service to the Iron Throne, the Freys are rewarded by Tywin Lannister. Walder Frey's second son Emmon Frey, married to Tywin Lannisters' sister, is given Riverrun. A Frey force of 2000 under Ryman Frey besieges Riverrun, which is held by Edmure's uncle Ser Brynden Tully "The Blackfish". They are aided by the Lannisters and the reluctant River Lords. The siege is poorly handled by Ryman, who has Edmure put on a gallows each day but never hangs him as he doesn't want to lose his hostage.
The River Lords despise the Freys. Many of them had relatives taken as hostages during the Red Wedding, or lost relatives and men. When Ser Jaime Lannister arrives to restore order to the Riverlands, he finds Ryman spends most of his time drinking and whoring. Edwyn attends a war council in place of his father, who is drunk. Edwyn suggests smearing night soil on arrows and killing Brynden Tully with them, as he thinks with the Blackfish dead the castle will fall. Lord Clement Piper then insults Edwyn, saying "You're a lying, treacherous weasel like all your kin. I'd sooner drink a pint of piss than take the word of any Frey." He inquires about his eldest son Marq, taken captive at the Red Wedding. Edwyn says he will remain at the Twins till Lord Piper has proved his loyalty. Clement inquires about the five knights who accompanied his heir to the Twins, nephews and cousins of his. Edwyn tells him some died and threatens to kill Marq as well if Lord Clement doesn't hold his "traitor's tongue." Piper challenges him to a duel, but Walder Rivers, a bastard great-uncle of Edwyn, offers to fight instead. Jaime stops the conflict, causing Clement to leave. Edwyn says they should send Ilyn Payne to execute him and that everybody who went to the Red Wedding was a traitor. Jaime tells him "The Twins took up the Young Wolf's cause as well. Then you betrayed him. That makes you twice as treacherous as Piper".
Jaime threatens to execute Edmure, causing Edwyn to come to stop him, at which Ryman comes over. Jaime criticizes Ryman for his stupidity, relives him of his command and sends him back to the Twins. Edwyn is given command of his father's force and told not to be as stupid as him, which he says won't be too difficult. Jaime persuades Edmure to convince his uncle to surrender Riverrun. However Edmure delays, giving the Blackfish time to escape.
When he is within a day's ride of the Twins, Ryman Frey, along with the three knights and twelve men-at-arms accompanying him, is hanged by the Brotherhood without Banners, making Edwyn first in line to become Lord of the Crossing. Edwyn suspects his brother Black Walder of being responsible, as now only he stands between them and the Twins. Walder Rivers says there is no proof and Black Walder is at Seagard, but Edwyn is still sure of it, thinking his brother has spies in the camp. Jaime notices he doesn't really mourn his father's death and suspects Edwyn is spying on Black Walder. He then asks for all the prisoners taken at the Red Wedding will have to be released, despite Edwyn's claim they will want recompensation. He then asks after Ser Raynald Westerling. Edwyn sneeringly tells him they are probably at the bottom of the Green Fork, having tried to set Robb's dire wolf Grey Wind free, before being wounded by quarrels and jumping into the river.