Handmaid (The Winds of Winter)

"They’re already here my Lord."

- Arya speaking of Walder Rivers and Lothar Frey in the pie.

The handmaiden is merely Arya Stark's disguise in the Season 6 finale. She is played by guest star Sabrina Bartlett and she only appears in "The Winds of Winter."

Season 6
Arya uses a skinned face from the Hall of Faces to dupe Walder Frey during his feast at The Twins, in which he has invited Jaime Lannister. During Bronn and Jaime's conversation, the handmaid serves the men while she keeps an eye on Jaime. Bronn believes this to be because she is interested in the Kingslayer, not realizing it is Arya staring down an enemy.

Later, after the feast has ended and the hall has emptied, the handmaid nears the dais where Walder Frey now sits alone. She sets down a pie for him as he mentions she is not one of his. He goes on to say she is pretty, before slapping her bottom, a common trait in the Frey man. It is then he begins asking where his "damn moron sons" are, before the handmaid replies "they're here my Lord". Slowly, the handmaid begins to expose the pie to the confused Walder and shortly after realizing his sons are in the pie itself, Arya exposes her true identity by peeling back the skinned face. She then slits his throat.