Maisie Williams character came in with an explosive episode intro this week. As we watch as she takes revenge on the Frays for what they did to the Starks at the red wedding. As you see her posing as Walter Fray, you can see how she's become an excellent assassin. I can't help but compare her to Cersei Lannister though. Both Arya and Cersei are proficient at killing, yet their motives are completely different. The character development for Arya Stark has come a long ways.
I feel her character and all the stark children will have an amazing role in the finally of the show.