Stalker is a young female Scyther who was part of Shadowdart's very first group of adolescents to instruct in the ways of the Code as Leader and a major catalyst for his descent into fanaticism and insanity.
It is often said that rape is about power, and the one in The Fall of a Leader is no exception. In fact, all the interplay between Shadowdart and Stalker is about power from beginning to end.
Shadowdart notes once in part VI that she reminds him of himself, and that is no coincidence; both of them share a thirst for and attraction to power thanks to their inherent insecurities. She, however, has done a much better job of attaining that power for herself so far than he had at her age, largely because while he was stuck on dueling ability and Leadership, she found power in the ability to manipulate and play mind games, overpowering others mentally instead of physically, and with her talents at this has built up something of a shield of confidence around her less confident self.
So Stalker is attracted to Shadowdart because he is the Leader, which both means he is physically powerful and gives him power over the other Scyther through his role. He, meanwhile, is attracted to her confidence and mental agility. But instead of this mutual attraction manifesting itself in any semi-healthy way, they both end up doing the only thing they really know how to do - try to overpower the other. She challenges him to repeated battles of wits during the First Prey lessons and is egged on when she feels that she is affecting him, and he frantically defends, becoming paranoid of facing 'defeat' at the hands of this adolescent - which, naturally, is a defeat in itself, since if he fears her, she has won. He has momentary triumphs in their mental tug-of-war, but he feels himself losing and begins to slip over the edge in the process. She also feels herself winning, delighting in having a degree of power over the Leader himself.
Finally, in the autumn, she approaches him in the forest, hoping to overpower him in the most definitive way possible: to make him break all his own rules and mate with her on her terms. Despite her best efforts, he refuses to give in, but thanks to her obvious influence on him, she is still considering it a victory for her when she begins to walk away. Then, as Shadowdart realizes she is about to slip away with yet another piece of his pride, it all twists itself around in his head: the only way for him to win is to have his way with her, and he proceeds to overpower her his way, through physical violence.
It's only a momentary victory, of course. In reality they've both lost: she's been violated and overpowered by him, while he's compromised everything he stands for as the Leader. The trauma and humiliation of the rape drive Stalker to suicide after a few hours' aimless wandering in the forest, and after a night of horror at what he's done, Shadowdart chooses to deny his responsibility and convince himself it was all her fault, cementing his descent into the very same hypocrisy and moral decay for which he once decried the previous Leader.
Stalker didn't exist in the original plan for The Fall of a Leader; in fact, it was supposed to end after part V. When I continued and made up the group of adolescents taking Shadowdart's lessons, this powerplay between him and the "tall female" emerged and from that moment, it could only end in tragedy. But it was very interesting tragedy to write, at the very least, and ultimately I think it made it a much better story with more power to stand on its own than the original idea. For that I'll always be grateful to Stalker, and she was very compelling to write while it lasted. Ultimately, her purpose in the story lies in her interactions with Shadowdart, hence why half of this bio is about him too, but I'd actually find it quite interesting to write more about her as an individual character of her own. Maybe someday.