Martyn Lannister

Martyn 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
Martyn, along with his brother Willem, 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 Martyn and his brother Willem are sons of Ser Kevan Lannister instead of being sons of one of Lord Tytos Lannister's brothers.