Fanny may be demure and kind and all of those qualities generally attributed to Princess Barbies; she's also pretty cute. But that's not to say that you can tell a book by its cover.
Because she's such a looker, Fanny has to deal with people gawking at her everywhere she goes. When she just walks into an inn, she attracts "the eyes of the host, his wife, the maid of the house, and the young fellow who was their guide; they all conceived they had never seen any thing half so handsome […] (2.12.2). Luckily, Fanny's got plenty of tricks up her sleeve to foil the bad guys. More than once, Fanny stands up for herself and gets herself out of a sticky situation.
But there's more. Fanny's smart and gorgeous, all right, but lots of her appeal comes from "a natural gentility, superior to the acquisition of art, and which suprized all who beheld her" (2.12.3). It's no surprise that Joseph would couple up with a nice young gal.
On top of that, Fanny has empathy in spades—perhaps more than any other character in the book. After the experience of watching a hare being hunted right in front of her, Fanny is "unable to assist it with any aid more powerful than pity; nor could she prevail on Joseph […] to attempt any thing […] (3.6.4).
Fanny feels deeply for animals and humans, which could be because of her own vulnerability. Fanny's the victim of three random attacks during the book, each of which she escapes by the skin of her teeth. Now, part of that might have to do with women's status in eighteenth-century Britain. As a young peasant woman, Fanny is doubly at risk on the open road because of her class and gender. She miraculously makes it out of harm's way each time because her friends are almost equally quick help those in need.
Spoiler alert—stop reading if you haven't made it all the way to the end of the book. Well, it turns out that Fanny's sis is none other than the famous Pamela Andrews, star of Samuel Richardson's novel of the same name. If we know one thing about Pam Andrews, it's that she's more virtuous, kind, and empathetic than anyone in the world. No, that's not an exaggeration. So by virtue of being related to Pam, Fanny's got to be pretty great.
Best of all, figuring out who Fanny's true parents are helps solve one of the smaller mysteries of Joseph Andrews. Fanny's "natural gentility" is repeatedly emphasized, but it's not totally clear where it comes from (2.12.3). Since Fanny's related to Pamela, all the puzzle pieces come together.
The point isn't so much that noble blood makes you a good person even if you've been kidnapped at birth. It's more of a metaphor: everyone has a lot of goodness and nobility in them if you look for it, so class differences, in the end, are surface-level and totally artificial. That makes it all the more awful that social status determines so much of a person's life in this world. Have things changed much today?