Kitty Frey

Kitty Frey is a minor character in the sixth season. She debuts in the episode "Blood of My Blood". She is the ninth wife of Lord Walder Frey.

Background
After the death of his eighth wife Joyeuse Erenford at the Red Wedding, Walder Frey takes the young Kitty to wife. At the Red Wedding, Walder had told Catelyn Stark, who ultimately killed Joyeuse in response to the death of her son Robb Stark, he would do just that.

Season 6
Kitty nervously stands by his side as Walder angrily berates his sons Lothar Frey and Black Walder for allowing Catelyn's uncle Brynden Tully to retake Riverrun. As his sons report to their father of the various threats to their hold on the Riverlands, Walder slaps Kitty on the rear and shouts that if he wants excuses, he would put her in charge. As he tells his sons what to do about the Blackfish, he pulls Kitty on to his lap and holds her close, making her visibly uncomfortable.

In the books
Walder Frey’s eighth wife, the teenaged Joyeuse Erenford, died at the Red Wedding in the TV version but not in the books. Therefore, Kitty, who he got to replace her is an invention of the TV series.

In the books, Catelyn’s last desperate act was to hold one of Walder’s lackwit (mentally disabled) grandsons hostage with a knife to his throat, but Walder bluntly said she could go ahead and kill him because he wasn’t of much use (displaying that he hypocritically doesn’t really value his own family members). Rather than introduce too many new characters, the TV version just streamlined this so Catelyn grabbed his wife Lady Joyeuse instead, and ultimately killed her as Walder said she was replaceable.

Thus, TV-Joyeuse's replacement appears in Season 6.

Her name wasn't stated in dialogue but was given as "Kitty Frey" in the Season 6 HBO Viewer's Guide website. Her background, however, is unknown, such as what her maiden name was.