Willem Lannister is a scion of House Lannister. His father is Kevan Lannister and his mother Dorna Swyft. He is a squire. Lord Gawen Westerling, a bannerman of his uncle, once suggested that Willem or his twin Martyn would marry Gawen’s daughter. Kevan refused this offer because he considered Jeyne of dubious blood due to her mother’s lineage.