I actually know why she is in the series, but still find her one of the most annoying characters in it. She could be named as the one that is protected and favored by Oscar. Or we could say that Oscar is some kind of godmother to her.
She is important for the story, because she is the link to something that Oscar doesn’t know and with her she finds out, in what conditions actually people live. For a moment Rosalie is also in love with her, but later she maries with Bernard, who is the one that explains about how the people suffers and the ideas of the third state.
The two women actually in some way help each other learn things about their deferent worlds, Rosalie find out that Marie Antoinette isn’t what people say she is, and on the other hand Oscar comes in contact with the revolutionary minds and similar.
It’s Rosalie’s husband that helps to free Oscar’s soldiers when they are accused of being traitors and she is also there when she dies. In a way, Rosalie is the one that keeps Oscar and André alive.