The Resistance Agents are elite members of the Resistance, whose existence was there to boost resistance morale. However the Agents became something else, they became a symbol of the resistance. These Agents are rarely mentioned, and even more rarely seen. Resistance members see that Agents never die, because they are legends. No one really knows who the agents are, but they do have a lot of enemies with the Combine, namely Overwatch Black Ops.
List of Known Agents
The Agents are seen as Half-Life's answer to Halo's Spartans.