The Hedge Knight (appears)|
The Mystery Knight (mentioned)
When a place opened in the Kingsguard, Daeron II Targaryen gave the position to Ser Willem. Either Daeron did not know, or willfully ignored his father, Aegon IV's, promise to Ser Quentyn Ball that he would be raised to the Kingsguard. This was one of the contributing factors to Ser Quentyn siding with Daemon Blackfyre during the First Blackfyre Rebellion.
Tourney at Ashford Meadow
Ser Willem, along with his two fellow brothers of the Kingsguard, Ser Roland Crakehall and Ser Donnel of Duskendale, escorted the royal Targaryen princes to Ashford for the tourney in 209 AC. After Prince Aerion Targaryen invoked the trial of seven all three Kingsguard members present were made part of his seven. Ser Willem was carried off the field unconscious at the end of the trial.