The Waif is a character that was well-written but I don't think many liked her in the story. She works for the Faceless Men and doesn't like Arya Stark at all.
When Arya Stark goes blind, she uses this as a way to torment her more.
Then when Arya gets her sight back, Arya betrays the Faceless Men's code which allows The Waif a free pass to murder her. My wife freaked out at this scene where The Waif chases Arya Stark down the streets of Braavos as she was so scared for Arya's welfare.
The Waif was clearly Arya's superior in skill, but turns out she can't fight in the dark. Whoops...