Last year when watching episode eight of season six titled "no one." When I finished watching it I was in shock since I was under the impression that Arya Stark had just been murdered. I go online to view the reactions of the fans and I can't seem to find anyone who believes in it. The top "Game of Thrones" critics on YouTube who do complex analysis of the show claim that this was the worst episode in the shows history due to its many plot holes. There is one article I found that mentions this theory, but doesn't give an explanation as to why it's possible: https://www.wired.com/2016/06/crazy-game-thrones-theory-ill-bet-right/
I'll go one to give reasons as to why it only makes sense that Arya has been killed.
Some of the critics claimed that the conversation with Jaqen and Arya at the end made no sense:
Also, how can Arya be walking around after the battle with the waif despite her many wounds?
Why the she go back to the temple of black and white?
Why does she mount the face of the Waif on the wall?
Why does Jaqen tell her she's "no one," but smiles when she says she's Arya and walks away without a care in the world?
The critics on Youtube that I've seen so far have said that Game of Thrones dropped the ball. There are many plot holes and none of it makes sense. My belief is that they didn't. The Waif killed Arya and is wearing her face and I'll attempt to prove it.
If you go back and rewatch the "training" of Arya you'll realize that Arya opens up to the Waif and reveals everything about her. In this scene Arya tells her not only her childhood, but also her deepest secrets and motives for her kill list:
If you read the books it goes into a bit more detail as to how being a faceless man is much deeper than simply acting. It reminded me the testimonies from undercover cops. After years of playing a role you start to believe that's who you are, develop really emotional attachments to people, but yet retain the knowledge that you're pretending in order to carry out a mission. But it is said that the faceless men use acting techniques when impersonating someone.
“Years of prayer and sacrifice and study are required to work a proper glamor.” “Years?” she said, dismayed. “If it were easy all men would do it. You must walk before you run. Why use a spell, where mummer’s tricks will serve?” (FFC Arya II)
So we know for a fact the faceless men use acting techniques when impersonating someone. If you look at the video again, you must ask why the hell would the Waif be wanting Arya to tell her such private information about her past, her list, her motives. The Waif presses on to understand Aryas psychology and attempts to uncover her motives. Based on the information provided by Arya to the Waif, the Waif could realistically "roleplay Arya."
Now, lets look at the next scene. The Waif tells Arya "You'll never be one of us, Lady Stark" to which Jaquen tells Arya that the Waif "she has a point"
Whole clip, start at min 10:30:
Jaqen then walks her to the room with all the faces, and he tells Arya a little about the history of the faceless men. He says that the faces on the wall are the original faces of past faceless men before they started wearing other faces. "These were the faces they wore in life, when they were not wearing others." Jaqen explains.
Let me repeat. The faces on the wall are the original faces the faceless men were born with, which were put in the wall when they started taking the faces of others aka became "no one."
They can't wear their original face and that of another at the same time, they must first become "no one" (Arya became blind from wearing a face when not being "no one"for some reason). But it's stated very clear that the faces on the wall are the original faces of the faceless men, which were placed on the wall when they were wearing the faces of others.
Therefore we have now a completely logical explanation for the face of the Waif in the wall:
Why did Arya return to the temple of black and white? Why did Arya lay the face of the Waif into the wall? Why the Jaqen congratulate Arya for becoming "no one"? What happened to her wounds? None of this makes sense if it's Arya who we expect to have won the fight.
In the other hand, based on the information provided by the show, it makes complete sense that the face of the Waif is hanging in the wall because the Waif is now wearing the face of another. Why did Jaqen tell "Arya" she's finally become "no one."? It's the Waif he's talking to (who was also in training) who has placed her original face in the wall because she's now wearing the face of Arya Stark.
The Waif knows all of Aryas deepest motivations, secrets and past, and therefore would realistically "play the role of Arya Stark." Based on information provided by the show, we have an explanation as to why why Waifs original face is in the wall, and why Jaqen congratulated "Arya" into being "no one."