Garth Greenfield is the Knight of Greenfield and head of House Greenfield.
Ser Garth is part of the Lannister host besieging Riverrun. He is taken captive during the battle in the Whispering Wood.
Ser Garth is held captive at Raventree Hall.
Ser Jaime Lannister goes to Raventree Hall and negotiates the surrender of Lord Tytos Blackwood. Garth is presumably freed from captivity.