From A Wiki of Ice and Fire
Martyn Lannister is a member of House Lannister. He is the third son of Ser Kevan Lannister and Lady Dorna Swyft and twin to Willem. He is a squire.
A Clash of Kings
Martyn is a part of the army gathered by Ser Stafford Lannister at Oxcross. He is among those captured when King Robb Stark and his army take the Lannister force by surprise.
A Storm of Swords
In exchange for the freeing of Robett Glover at Duskendale, Robb has Rolph Spicer release Martyn at the Golden Tooth.
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