Jeyne Poole is the Sansa Stark friend forced to pose as Arya Stark in the novels but absent from Game of Thrones.
Jeyne is the daughter of Vayon Poole, a steward of House Stark, and grows up alongside Sansa at Winterfell. Unlike Sansa and Arya, she is not a member of the Stark family, which makes her substitution possible but emotionally devastating.
After the Stark defeat, Jeyne is taken into Lannister control and eventually delivered to the Boltons. Because the real Arya is missing, she is presented as Arya and married to Ramsay Bolton to help legitimize Bolton rule over the North. Theon Greyjoy recognizes that she is not Arya and sees the severe abuse she has suffered.
The television adaptation transfers this storyline from Jeyne to Sansa, who is married to Ramsay instead. That major change is why viewers sometimes remember Sansa as the novel character in this plot. Jeyne Westerling is a different person: she marries Robb Stark, while Jeyne Poole is the false Arya.