Merrett Frey was the ninth child of Lord Walder Frey of the Twins and grew up in the competitive and crowded castle where numerous factions plotted against one another, all encouraged by their father. He was large and skilled at fighting, and seemed to have the makings of a knight. Merrett became a squire in the service of Lord Sumner Crakehall in the Stormlands. He served alongside Jaime Lannister as squires, but he had a habit of bullying other squires, until he unsuccessfully tried to bully Jaime; it is implied that Jaime turned the tables on him.
He served in Lord Sumner's campaign against the Kingswood Brotherhood, but did not fare well. First he caught a pox from a whore and spread it through the camp. This kept him out of battle for quite awhile. Then finally getting back, Merrett was captured by one of the outlaws, a woman known as Wenda the White Fawn, and had to be ransomed back by Lord Crakehall. Before returning him, Wenda had her sigil a brand of a white fawn burned into his buttocks. In his first battle after being ransomed, Merrett Frey took a blow to the head from a mace that nearly killed him and left him in permanent pain with migraines for the rest of his life. Becoming a knight became impossible. He took to drink to dull this pain. He was thought at first to be lucky in marriage as he married a daughter of House Darry, but they lost much of their prestige and power after the fall of the Targaryens. Merrett had four children. One called Fat Walda Bolton is married to Roose Bolton and another Amarei Lannister is married to Lancel Lannister.
Merrett was included in the conspiracy to carry out the Red Wedding and was assigned to get Greatjon Umber too drunk to effectively fight, although he failed in this task. He later asked to be assigned to deliver the ransom for Petyr Frey, called Pimple, captured by the Brotherhood Without Banners. He was trying to prove himself reliable in a crisis and worthwhile to the new heir of House Frey. But Petyr had already been hanged and the outlaws held Merrett to be one of those responsible for the Red Wedding, the Brotherhood and Lady Stoneheart condemned him to death by hanging.
While at Lancel's new estate, that formerly belonged to the House of Darry, Jaime Lannister remembers Merritt Frey as being a bully that he didn't like.