From A Wiki of Ice and Fire
|Book(s)|| A Feast for Crows (Appears)|
A Dance with Dragons (Appears)
The waif is a priest of the Many-Faced God in the House of Black and White. She does not give her name. She wears a robe with a cowl of black on the right side white on the left side. She appears to be a young girl, however she is 36 years old.
The waif appears to be a pale little girl with a gaunt boney 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 daughter would inherit the waif's father's wealth. The stepmother approached the Many-Faced God, but could not bear the sacrifice he 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.
She appears to be a child because of her dealing with poison.
A Feast for Crows
The waif is assigned to teach Arya the tongue of Braavos and then how to detect lies. Arya is to teach her the Common Tongue of Westeros. They then play the lying game. When Arya returns to the House of Black and White on the new moon she helps the waif with preparing poisons and the waif explains the different poisons to Arya. The waif gets Arya the warm milk the kindly man requests for her. The next morning Arya wakes up blind.