From A Wiki of Ice and Fire
Ser Lomas Estermont is a knight of House Estermont and is the father of Ser Andrew Estermont.
A Clash of Kings
Ser Lomas is listed as supporting Stannis Baratheon at the onset of the War of the Five Kings.
A Storm of Swords
Ser Lomas is among those who remain loyal to Stannis after the Battle of the Blackwater.
A Feast for Crows
Ser Lomas is part of the force defending Storm's End from the forces of King Tommen I Baratheon.
The Appendix of A Feast for Crows states Lomas is the uncle of Stannis Baratheon, the father of Andrew Estermont, and the brother of Eldon Estermont. However, the Appendix of A Dance with Dragons states Lomas is the cousin of Stannis and the son of Eldon. See House Estermont/Family Trees.