The Trenchcoat Soldier, also known as the Trenchcoat Guy by Griggs and Big Daddy by MacCready, is the leader of the Hydra 5 Base and sends Catherine Roth to scout at the St. Neal resistance base. He is a high ranking Combine soldier.
He is important to the chapter Empty Coast, in which he is the main antagonist. He directs the Hydra 5 Base, which is greatly involved with the St. Neal Energy Plant. The Trenchcoat soldier sends Catherine Roth to investigate MacCready's base in St. Neal and she is killed. When he hears word of this, he sends three waves of soldiers at MacCready's base, where most of the people there are killed, as a small squad including Gordon and MacCready. The Trenchcoat Soldier also killed Sandy in a falling out where Sandy offered a truce, which was actually a lie, but he knew that they wouldn't offer a truce so he killed them, since they were rebels.
The Trenchcoat Soldiers seems to be smarter than other Combine soldiers, as he is a director of the the Hydra 5 Base and also knows if people are lying to him, like Sandy.
Dan's Half Life - Empty Coast