|Born||In or between 285 AC and 287 AC|
|Died||In 299 AC, at Riverrun|
|Book(s)|| The World of Ice & Fire (appendix)|
A Game of Thrones (appendix)
A Clash of Kings (mentioned)
A Storm of Swords (appears)
A Feast for Crows (appendix)
A Dance with Dragons (appendix)
Lord Gawen Westerling, a bannerman of his uncle, once suggested that Willem or his twin Martyn would marry Gawen's daughter, Jeyne. Kevan refused this offer because he considered Jeyne of dubious blood due to her mother's lineage.
A Game of Thrones
Willem is part of the army of his cousin, Jaime Lannister, that invades the Riverlands. He fights in the Battle of the Whispering Wood and is captured there together with another cousin, Tion Frey, by Robb Stark.
A Clash of Kings
A Storm of Swords
Lord Rickard Karstark's sons were both killed by Ser Jaime Lannister in the Battle of the Whispering Wood. Rickard had wanted to kill Jaime, but when he was released by Catelyn Tully he felt robbed of his revenge and choose Willem and Tion instead. With some of his men, Rickard kills the boys and their guards. One of the boys is killed in his sleep, while the other had tried to fight before he was killed.
Robb Stark is furious about the killing of the captives. He executes Rickard with his accomplices.
After they have been murdered, Catelyn Tully sees the bodies of Willem and Tion Frey. She observed that one was blond, wore his hair long, and was trying to grow a beard. He was probably killed in his sleep. His cousin was brown-haired and had fought for his life. Both were pale from their imprisonment.