After spending her youth as the mostly unwilling mistress of a rich aristocrat, Nastasya chafes under her status as a fallen woman and wrestles with the decision of whether to marry Ganya, Rogozhin, or Myshkin.
Let's face it, folks. There is probably almost no way that you could read this book and have a positive impression of Nastasya. And really, for some of the novel's critics, she is just the days-of-yore version of the stereotypical femme fatale—you know, the super-hot bad woman who lures the unsuspecting hero to his doom because her hotness is just so hot that he can't possibly resist her. Which maybe isn't the most positive spin on female sexuality that we could think of, but hey, them's the old-timey breaks. Not that they don't have a point. After all, in her first appearance in the novel, we see her bargaining with Rogozhin over her price:
Eighteen thousand roubles, for me? Why, you declare yourself a fool at once," she said, with impudent familiarity, as she rose from the sofa and prepared to go. Ganya watched the whole scene with a sinking of the heart.
"Forty thousand, then—forty thousand roubles instead of eighteen!" [said Rogozhin.] Nastasia Philipovna continued to laugh and did not go away. […] "Very well then, a hundred thousand! a hundred thousand!" […]
"Surely there must be someone among all of you here who will turn this shameless creature out of the room?" cried Varia, suddenly. She was shaking and trembling with rage.
"That's me, I suppose. I'm the shameless creature!" cried Nastasia Philipovna, with amused indifference. (1.10.31-42)
So yeah, seemingly it's all here—she is basically just prostituting herself out for a whole bunch of money in front of an audience.
But honestly, if that's all this character was, this novel wouldn't be nearly as interesting—and the prince's motivations would be so much clearer and less frustratingly indecipherable. Think about it: if Myshkin exists in the novel to be the prototype of the "perfectly beautiful man" then this idea would break down pretty quickly if immediately upon meeting this woman he lost himself to lust.
But, if that is not what is driving him, which we know isn't since they live together for months but don't have sex, then maybe we need to reconsider just what Nastasya is doing in the text. We could argue that Nastasya is really not a femme fatale (and we're not talking about the Britney album here) type at all, but instead a deeply insightful psychological portrait of a pretty damaged person. This part of her character comes through most clearly in her interactions with Myshkin, who can really see her and not the sex bomb that everyone else sees.
From the very first time he sees her portrait—her miniature portrait!—he is immediately onto her dual nature: "I feel sure that her destiny is not by any means an ordinary, uneventful one. Her face is smiling enough, but she must have suffered terribly—hasn't she? […] It's a proud face too, terribly proud! And I—I can't say whether she is good and kind, or not" (1.3.109). Together, he and she spend half of the novel trying to make her be good rather than bad, to fix whatever parts of her are broken.
How is she broken? Let us count the ways: (1) raised to be a gross old dude's mistress from a young age without having any say in the matter; (2) feels horrible about herself that she didn't try to escape from that life (although, realistically, where would she have gone in that day and age?); (3) torn between viewing herself as society views her—a fallen, irredeemable woman—and trying to live a normal life just to show everyone up; (4) addicted to the attention that her victimhood and outsider status give her; (5) hell-bent on self-destruction and seems like she spends the novel just psyching herself up to be harmed by the clearly violent Rogozhin. Is that enough? Yeah, that's probably enough.
What is really fascinating is just how very detailed and highly specific all this is on Dostoevsky's part. It's almost like he had a person like this to study before writing this character. Oh wait, yeah, he totally did. Nastasya is actually based on Polina Suslova, a woman that Dostoevsky had an affair with. He was 40 to her 21, and she was a very difficult person. Still, that didn't stop him from proposing to her after his first wife died, and being totally mystified by her refusal, which came after years of asking him to leave his wife for her. Huh.