Ser Helman Tallhart is the head of House Tallhart and the Master of Torrhen’s Square. It is unknown how Ser Helman became a knight. Either he was a follower of the Faith (which is uncommon in the North save for House Manderly), or he was knighted for a great act of bravery, most likely during the Greyjoy Rebellion.
When Robb Stark calls the Northern banners in response to Lord Eddard’s arrest, Ser Helman leads the Tallhart men to join the army at Winterfell, leaving his brother Leobald in command at Torrhen’s Square. He is part of the host that marches south to Moat Cailin. Robb Stark names him as the commander of the garrison of 400 archers and swordsmen left at the Twins to ensure the loyalty of House Frey.