Near the end of A Storm of Swords, we learn that, as part of his alliance with the Lannisters, Roose Bolton has been made Warden of the North, and his son Ramsay has been legitmized and betrothed to Arya Stark. We also meet the fake, imposter Arya that the Lannisters are sending north to marry Ramsay. But who is this strange girl? Thus far, we only have one suspect who fits, but the clues are compelling.
That suspect is Jeyne Poole. Daughter of the steward of Winterfell, Jeyne was one of Sansa’s closest friends. She traveled south to King’s Landing with Sansa and her father, but after the fighting after Robert’s death, she is not heard from again. But why would we believe that she is the fake Arya Stark?
First, let’s take a look at what happened to her after the fighting. Jeyne was thrown into a room with Sansa when the fighting began (GoT, pg. 543), but Cersei becomes angry when she learns of this. Apparently she wants Sansa to be alone, so she doesn’t hear what really went on during the fighting. An interesting quote:
Queen Cersei looked at each of the councillors in turn. “I won’t have Sansa fretting needlessly. What shall we do with this little friend of hers, my lords?”
Lord Petyr leaned forward. “I’ll find a place for her.”
“Not in the city,” said the queen.
“Do you take me for a fool?”
The queen ignored that. “Ser Boros, escort this girl to Lord Petyr’s apartments and instruct his people to keep her there until he comes for her.”
--GoT, pg. 547
And that’s the last we hear of poor Jeyne Poole--being sent off to Littlefinger’s apartments for some unknown purpose. All of this might have been forgotten except for another quote, when Tywin is telling Tyrion about the fake Arya:
“...and Roose Bolton becomes Warden of the North and takes home Arya Stark.”
“Arya Stark?” Tyrion cocked his head. “And Bolton? I might have known Frey would not have the stomach to act alone. But Arya...Varys and Ser Jacelyn searched for her for more than half a year. Arya Stark is surely dead.”
“So was Renly, until the Blackwater.”
“What does that mean?”
“Perhaps Littlefinger succeeded where you and Varys failed. Lord Bolton will wed the girl to his bastard son.”
--SoS, pg. 595
Tywin’s above quote heavily implies that it was Littlefinger who produced the fake Arya. Since Jeyne was last seen being sent to Littlefinger, it is not too much a stretch to assume that Littlefinger is simply sending her elsewhere.
Jaime has the opportunity to meet the fake Arya:
A groom led a fine gray mare out a stable door. On her back was mounted a skinny hollow-eyed girl wrapped in a heavy cloak...The girl’s long brown hair blew wild in the wind. She had a pretty face, he thought, but her eyes were sad and wary.
When she saw him, she inclined her head. “Ser Jaime,” she said in a thin, anxious voice. “You are kind to see me off.”
Jaime studied her closely. “You know me, then?”
She bit her lip. “You may not recall, as I was littler then...but I had the honor to meet you at Winterfell when King Robert came to visist my father Lord Eddard.” She lowered her big brown eyes and mumbled, “I’m Arya Stark.”
Jaime had never paid much attention to Arya Stark, but it seemed to him that this girl was older...
--SoS, pg. 820
We can pull loads of interesting facts from this one description. For one, Jaime thinks the fake Arya is older than the real Arya. This makes sense, because Jeyne was the same age as Sansa (GoT, pg.472), about two years older than Arya. Also, the fake Arya is a pretty northern girl with dark hair. Jeyne is described as a ‘pretty, dark-haired girl.’ (GoT, pg. 472) It does not seem very likely that there are two pretty, dark-haired northern girls who are a few years older than Arya that happen to be in Littlefinger’s custody, so the odds are that it is Jeyne. Additionally, the fake Arya seems sad. Of course, this could be because she is marrying the monstrous Ramsay Bolton, but it could also be because her father was killed and she was taken away from her friend. Finally, the girl recognizes Jaime straight off. This too makes sense, because Jeyne would have noticed Jaime from when he visited Winterfell.
It makes sense that Jeyne would be picked to masquerade as Arya. She has the benefit of knowing Winterfell and its inhabitants, as opposed to just some random orphan who could be asigned to play the part. So while the evidence is nowhere near conclusive, Jeyne Poole is the only suspect we have at present for being the fake Arya. Her physical description and history match up, and all the clues point in that direction. GRRM has stated that we will eventually discover what happened to Jeyne (SSM, # 50), so we shall just have to wait and see if our theory is correct.