|Book(s)|| A Game of Thrones (mentioned)|
A Storm of Swords (mentioned)
A Game of Thrones
While Eddard Stark is out on a ride with King Robert I Baratheon, the king asks for the name of his bastard's mother, and Eddard replies that her name is Wylla. Eddard then states he does not want to talk about it. Robert then adds that she must have been quite a woman, if she was able to make Eddard Stark forget his honor even for an hour.
A Storm of Swords
Lord Edric Dayne tells Arya Stark that he and Jon Snow are "milk brothers". He elaborates that one of the servants of his house, called Wylla, was Jon Snow's mother. When his own mother had no milk, Wylla became Edric's wet-nurse. Arya, who has never heard the name and is aware that Jon doesn't know anything about his mother, first thinks that Edric might be mocking her, but when he swears on the honor of his house that he's telling the truth, she decides to remember the name, so that she can tell Jon when they meet again.