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.
The Waif was clearly Arya’s superior in skill, but turns out she can’t fight in the dark. Whoops…
I don’t remember reading about her in the books. Then again, there were so many characters to keep straight, I probably did and just forgot about her.