Appearance and Character:
Willem Lannister is the son of Ser Kevan Lannister and Dorna Lannister, he is the twin brother of Martyn Lannister. He has two other siblings, an older brother, Lancel Lannister and a younger sister, Janei Lannister.
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.