Willem Lannister

Willem Lannister is a character in the third season. Along his brother Willem, he's a squire in the Lannister army. Both descen from one of the brothers of the late Lord Tytos Lannister.

Season 3
Willem, along with his brother Martyn, is captured by the forces of House Tully led by Edmure Tully following the Battle of Stone Mill. He's kept hostage at Riverrun.

In the books
In the A Song of Ice and Fire books Willem and his twin brother Martyn are sons of Ser Kevan Lannister instead of being sons of one of Lord Tytos Lannister's brothers. He's captured at the Battle of the Whispering Wood.