Symbolism, Imagery, Allegory
Okay, you knew this was coming.
We can't overstate the importance of Miss Pamela Andrews to Fielding's work in general—and to Joseph Andrews in particular. We'd even say that she's less of a character and more of a symbol in this book. Joseph breaks it down for us: "I don't doubt, dear sister, but you will have the grace to preserve your virtue against all trials, and I beg you earnestly to pray, I may be enabled to preserve mine […]" (1.10.5).
Hold up. What's he talking about? Obviously, Joseph is trying to withstand the temptation to give up his virtue. But he's also talking about Pamela's trials to hold off her master, Mr. B. This battle of wills is the whole plot of Pamela, Samuel Richardson's masterpiece (or master-stinker, if you agree with Fielding).
The ghost of Pamela haunts all of Joseph Andrews. Joseph is constantly in his sister's shadow, trying to measure up to her unattainable level of perfection. Although we don't actually meet Pamela, the character, until the very end of the book, Joseph hopes to "copy [her] example" all the way through (1.10.5).
Isn't this all a little bit much? Surely, no one thinks about their bratty sibling that much. Still, we're talking about Pamela Andrews, the single biggest sensation to hit eighteenth-century literature. Even better, Fielding has a hay day making fun of the secret of the book's success. Despite Pamela's will to resist sex, people read the book because it was sexy.
On top of that, by switching the sex of the main character from a female (Pamela) to a male (Joseph), Fielding is sending up Richardson's whole premise. At least in the eighteenth century, it would have been a lot more absurd for a handsome young buck to be holding off a bunch of lusty ladies than for a virtuous maiden to be holding off a lecherous old man. Fielding is making fun of the whole double standard.