I have purposely not chosen a specific woman actor for this role, because I think Thalia should be played by someone completely new to audiences, mysterious even to them, never having played a “good” or “bad girl,” but exuding yonic power like a pheromone. In a narrative populated with characters of mixed and unknown motives, hers are the most inscrutable.
Typically, a personal helper figure is the same gender as the hero/heroine, and I did follow that formula with Nexus and Sage. However, as William Wyrd is apprenticed to many masters in the course of his journey, it seemed as if a woman could teach him the things he most needed to know: patience, self-discipline, endurance of pain and deprivation–even yoga breathing! More than these things, however, he needed to be humbled, to strip away his ego constructs–show him that he could accept help, instruction, and not handle every task alone.
I also wanted her, physically, to be an opposite twin of Nexus: dark, young, exotic, and subtle, whereas my woman protagonist is fair, approaching middle age, sometimes naive, and an American farmgirl-turned-lawyer. For these reasons, Will is attracted to her, even while she trains him in skills that repel him.
Thalia takes on two distinct personas in the course of the novel: first, a woman soldier/assassin, specializing in intelligence and black ops. Second, as a feral seductress, who may or may not be the spawn of Will’s desert visions.