When you answer the question "Who are you?" with "No One" in caps for a long enough time, being No One takes on an identity of its own. In Arya's case it means trying and failing the same rigorous test by the waif time and time again.
The 20-year-old rising star discusses landing her first pro gig, the part of Sansa Stark, at 13, growing up in the public eye ("It can really get you down") and Sansa's season-six evolution ("She really showed people what she could do").