Merry Frey

Merry Frey is one of the many granddaughters of Walder Frey. She only appears in the Season 3 episode "The Rains of Castamere".

Background
Merry is one of the many granddaughters of Walder Frey, the Lord of the Crossing.

Season 3
Along with several of Lord Frey's other daughters and granddaughters, Robb Stark had the option of marrying her. When he arrives at the Twins for his uncle Edmure's wedding, Robb apologizes for his slight against them. When introducing Merry, Lord Walder calls her Wertha, Walra, and Waldina before she meekly corrects him, though he quickly dismisses her. However, during the wedding feast she sits at the main table alongside Walder Frey's wife, Lord Walder himself, and the newlywed couple - the only one of Lord Walder's children (other than the bride Roslin) to sit as his table during the wedding.

In the books
In the A Song of Ice and Fire novels, there's a granddaughter of Lord Walder named Merianne Frey, who is also known as Merry, daughter of Ser Whalen Frey and Sylwa Paege. Whalen is the third child of Lord Walder's fourth wife, Alyssa Blackwood. Lame Lothar Frey is Walder's eldest child with Alyssa, making him Whalen's older brother and Merianne's uncle.

Merianne is only eleven years old in the books.

Merry was one of the Frey women presented to Robb Stark when he came to the Twins for Lord Edmure Tully's wedding.