Merry Frey

The pact Lady Catelyn Tully struck with Lord Walder Frey left Robb Stark free to marry any of the lord's daughters or granddaughters, and Merry was one of them. The agreement was never honoured: Robb married someone else, and when he arrived at the castle for his uncle Edmure's wedding to one of Walder's daughters, he had to apologise in person to the women of the family.

When it came to Merry's turn, Lord Walder mixed her up with his other granddaughters, calling her by several wrong names before she quietly corrected him herself. At the wedding feast that followed, she sat at the high table beside Lady Joyeuse Erenford, her grandfather, and the newlyweds.

Later in the war, Merry appears again in the hall of the Twins, enjoying a family celebration while, nearby, Lord Walder harshly reprimands two of his sons, Walder Rivers and Lothar Frey.

At another banquet, Merry is one of the women of the house pouring wine for the men of the family. She watches in horror as the poison takes hold and the Freys collapse one after another, the moment it becomes clear that the man sitting in Lord Walder's place was Arya Stark, wearing his face.

There is no character named Merry Frey in A Song of Ice and Fire. The wiki points to a possible counterpart in Merianne Frey, nicknamed Merry, daughter of Ser Whalen Frey and Sylwa Paege, and Lord Walder's granddaughter through that line of the family.