Lymond Goodbrook is the head of House Goodbrook. He is a good friend of Ser Edmure Tully.
Lord Lymond is among the nobles from the riverlands who accompany Lord Edmure Tully to his wedding at the Twins.
After the Red Wedding, Lymond is among the riverlords pardoned by King Tommen I Baratheon and is subsequently welcomed back into the king's peace.
Lord Lymond is presumably one of the riverlords present at the siege of Riverrun since Ser Jaime Lannister sees the Goodbrook banner when he arrives at Riverrun.