Why do I love Rosalie so? Let me count the ways...
1. She's just so drop-dead sweet. In some characters this can be nauseating; on her, I find it adorable.
2. She's so... slashy. She is clearly (and canonically) in love with Oscar, although she's perfectly aware that Oscar is a woman, and she spends quite a bit of time gushing over how fine Antoinette is, too.
3. Those eyes! They are surely bigger than any human's eyes should be!
4. She makes for wonderful yuri fanfiction, if that's your thing (see point #2).
5. She's all gentle and devoted and yet determined and strong also; see that whole arc where Oscar tries to teach her to ride and fence and shoot so she can be revenged on the woman who killed her mother.
6. She's a noblewoman who doesn't know it because she was raised by a loving adoptive mother in poverty. So very fairy-tale-ish... and there is something of a fairy-tale heroine quality about her.
7. Because she's from a poor background, she opens Oscar's eyes to the plight of the French people, and all that, which is a fairly important plot point.
8. She's practically the only character to survive to the end of the series. In fact I think she IS the only female survivor we know about for sure.
9. Unlike Oscar, Antoinette, and half the court girls, she never gets a crush on Fersen, which is worth a point in my book. Although this may, arguably, be because she's a lesbian. ;)
10. Assuming she is lesbian/bisexual, she's wonderfully sparkly and feminine and unstereotypical. Yay for pretty big-eyed femme girls.