Waif

The Waif is a recurring character in the fifth and sixth seasons. She's portrayed by Faye Marsay and debuts in "High Sparrow". The Waif is an acolyte of the Faceless Men serving in the House of Black and White in Braavos.

Season 5
The Waif has a sour demeanor and low opinion of Arya Stark when she arrives at the House of Black and White. When Arya presumes to claim that she is "no one" - a skill that the Faceless Men only achieve through dedicated training - the Waif becomes angered and beats Arya with her cane. She overpowers Arya and beats her to the floor before she can react. Jaqen H'ghar then enters the room and makes her stop. Jaqen is also critical of Arya, but more gently explains that she cannot truly be "no one" yet because she still thinks of herself as "Arya Stark", wearing Arya's clothes and carrying Arya's remaining possessions. Later, Jaqen leads Arya into the basement of the building where the bodies of those who come to seek the mercy of a quick death are taken. The Waif is there, and knows her way around well enough to undress and wash the body. Arya begins to help her, and asks what they do with the bodies after they are cleaned, but the Waif gives no response.

When Jaqen sends Arya out on her first assignment - to deliver a gift of the Many-Faced God to the Thin man - the waif expresses doubt as to Arya's readiness and chance of success.

In the books
In the A Song of Ice and Fire novels Arya dubs this woman as "the waif" for her appearance. She appears to be a pale little girl with a gaunt bony face and dark eyes that look as big as saucers.

The waif was the only child of an ancient noble house, her father's heir. Her mother died when she was little. When she was six years old her father remarried. Her stepmother treated her well until she gave birth to a daughter of her own. Her stepmother then wanted the waif to die, so her own daughter would inherit the waif's father's wealth. The stepmother approached the House of Black and White, but could not bear the sacrifice asked of her. She then tried to poison the waif. She did not succeed. When the healer in the House of the Red Hands told the waif's father what the stepmother had done, her father offered a sacrifice of two thirds of his wealth and the waif to the Many-Faced God for his wife to receive "the gift" of the god - death. His prayer was answered and the waif came to the House of Black and White to serve.

The poison, however, stunted her growth, causing her small and boney appearance - she only looks like a young child, but is actually 36 years old.