Rose is the sweetest, loveliest, most virtuous young lady ever. She’s pretty much a stock Victorian heroine. She’s self-sacrificing, loving, kind to animals and small children, and blonde. She is occasionally prone to fevers, but doesn’t die of them. (Yep: that’s pretty much every early Victorian heroine, right there.)
But Rose differs from those other heroines in some interesting ways: first of all, she’s not aristocratic by birth. In fact, her birth is pretty questionable. They think, at first, that she is illegitimate, and then later they realize that even though she isn’t illegitimate, she still has a stain on her family honor (her sister got pregnant without being married).
Perhaps because of her own questionable background, Rose is able to sympathize with folks who are down-and-out in ways that other, more aristocratic, heroines might not. She’s the one who begs Mrs. Maylie to take Oliver in and protect him, and she’s able to pity Nancy, rather than condemn her. In response to Rose’s sympathy, Nancy says, "[...] if there were more like you, there would be fewer like me!" (40.57).