In many ways, Fanny is mystery. She dislikes the woman who is courting her cousin, but she doesn't try to stop her. She cares about her cousin, but never attempts to inform him of his love interest's mercenary tendencies. The obvious reason for this is that Fanny thinks that he knows already, but, in that case, what was he doing winning her affection? And why doesn't she tell anyone about her engaged sister's flirtation? The character appears extremely reticent, not even giving her reasons for not joining in the play everyone else puts on.
While Fanny appears unconventional, she adheres strictly to the rigid laws of society for women at that time. She is astounded by the behavior of Mary Crawford--not just because she is flirting with her cousin, but because she is improper.
Fanny rises in the reader's opinion when she emphatically refuses to marry the unprincipled Henry Crawford. While her will-power here seems to come out of nowhere, she has actually been preparing for this moment during the whole of the novel. Like the proverbial "bump on a log" Fanny refuses to budge, despite the fact that many of the characters see this marriage as a duty for her.
One of the most universal points about a typical heroine is that she is, or thinks of herself as, an unconventional heroine. She's headstrong; she's athletic; she's outdoorsy; she questions society's rules. Why shouldn't she be unconventional? Because "unconventional" has become a convention, and of course, all of the poor "unconventional" heroines have become conventional.
Which means when we meet a truly unconventional heroine like Fanny Price, we're sure surprised.