If there's one show out there right now that can generate a considerable number of conspiracy theories, it's HBO's Game of Thrones. Starting from 'R+L=J' to 'Tyrion is half Targeryen' to 'Bran = Three-eyed Raven', we've seen it all. And since the show is departing from the books in many ways, these theories are just getting more interesting.

One of the most intriguing theories we came across recently revolves around Arya Stark and the girl who's simply addressed as 'the Waif' (go read the books).

Source: b'Source - HBO'

Reddit user catNamedStupidity recently wrote about it in a post on the ASoIaF subreddit. 

So here are a few things which are interesting about the waif.

The waif is Arya. Arya is suffering from some sort of dissociative identity disorder. A case can be made that when Arya came to the House of Black and White, the water that she drank induced this in her. In a way, The Waif is no one. Her only mission is to kill off Arya, the inner Arya, the one who can never be one of them.
This is why Jaqen H'ghar always wants the Waif to leave the room before he begins talking to Arya. He essentially wants Arya to stop behaving in a dual personality manner. He just wants to talk to Arya. So the Waif leaves. Just when Arya is alone, she appears again.
When the waif asks Jaqen H'ghar, "You PROMISED", it's actually Arya. Arya has finally asked to kill her inner self. Al lot of people have observed that Jaqen H'ghar looks a bit pleased with himself when he permits the Waif.

Is... is Arya Stark pulling a Fight Club on us? Even though it's just a theory, it'd be freaking awesome if it were to turn out that way.

Source: Giphy

On top of that, Reddit user _JeRM also added this - 

I can see this happening in the show:
Arya and the Waif do battle, and after fighting valiantly, Arya is killed. We are all fucking stunned for a few moments (maybe even 30+ minutes if they decide to show other scenes elsewhere).
The Waif goes to Jaqen and announces that Arya Stark is dead. The waif removes her face to reveal that it was Arya all along, and Jaqen asks who a girl is.
A girl replies "No one." - Credits.

Damn.

You can read the entire discussion here.