"[Myshkin] is another alternative for me," said Nastasya, turning once more to the actress; "and he does it out of pure kindness of heart. I know him. I've found a benefactor. Perhaps, though, what they say about him may be true—that he's an—we know what. And what shall you live on, if you are really so madly in love with Rogozhin's mistress, that you are ready to marry her—eh?"
"I take you as a good, honest woman, Nastasya Philipovna—not as Rogozhin's mistress."
"Who? I?—good and honest? […] Oh, you get those ideas out of novels, you know. Times are changed now, dear prince; the world sees things as they really are. That's all nonsense. Besides, how can you marry? You need a nurse, not a wife."
The prince rose and began to speak in a trembling, timid tone, but with the air of a man absolutely sure of the truth of his words.
"I know nothing, Nastasya Philipovna. I have seen nothing. You are right so far; but I consider that you would be honouring me, and not I you. I am a nobody. You have suffered, you have passed through hell and emerged pure, and that is very much. Why do you shame yourself by desiring to go with Rogozhin? […] Nastasya Philipovna, I love you! I would die for you. I shall never let any man say one word against you, Nastasya Philipovna!" (1.15.46-62)
It's interesting that this passage really connects Myshkin with Pavlichev, the man who semi-adopted Myshkin as a child, and who is later described as having paternal feelings for sick or disabled children. Check out how Nastasya figures out that what Myshkin really wants is to be is her "benefactor."