Willem Lannister

Willem Lannister is a character in the third season. Along his brother Martyn, he's a squire in the Lannister army. Both are great-grandsons of the late Lord Tytos Lannister's brother.

He is fourteen years old in Season 3.

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. The two are kept as hostages at Riverrun, the seat of the Tullys.

After Robb Stark visits Riverrun to attend the funerals of Lord Hoster Tully, Willem and Martyn are visited in their cell by Queen Talisa Stark, who tends to Martyn's wounded hand. Willem looks in silence while Martyn asks if the tales told among the Lannisters about Robb turning into a wolf at night or devouring his enemies are true.

In the books
In the A Song of Ice and Fire novels, Willem and his twin brother Martyn are sons of Ser Kevan Lannister and younger brothers of Lancel Lannister instead of being great-grandsons of one of Lord Tytos Lannister's brothers. He and his cousin Tion Frey - son of Genna Lannister and Emmon Frey - are captured at the Battle of the Whispering Wood.