In "Oathbreaker", the third episode of the sixth season of Game of Thrones
Arya drinks the water from the fountain in the House of Black and White to gain back her eye-sight.
I haven't read the books, but from the series alone my understanding is that
only people who are truly "No One" can drink the water without being harmed. The Faceless Man also says something along those lines just before Arya drinks the water.
So what are the criteria for being truly "No One"?
In particular Arya still strives to kill the names on her list.
Is this maybe the purpose of the brotherhood? To become "No One" you have to give up everything except for a list of names that you intend to offer to the god of many faces? Like I said, haven't read the books, so correct me if I'm wrong.