The first mention of Agent itself is at page 807 (paperback version) of Wizard's First Rule the following diagloue takes place between Darken Rahl and First Order Wizard Zeddicus Zu'l Zorander as Darken is being sucked into the blackness of the abyss of the Boxes of Orden.
Darken Rahl's voice came hollow, empty, dead. "Read the prophecies, old man. Things may not yet be as final as you think. I am an agent."
This reference to an agent could mean one allied to the Keeper and doing his bidding in the world of life. The Sisters of the Dark would be such agents.