I agree that Mary knows how to use her personality and wittiness opportunistically to charm people even though she is not always successful in getting what she wants. Yet, I do feel as though I should give her credit for a sincere and genuine concern for Fanny and that Mary's gesture is an instance of kindness on her part that worked for her benefit. When Mary is just being Mary without any agenda, that is when she is most attractive such as when she first arrives and Edmond is attracted to her when she is not really trying to engage him romantically but merely as a companion to ward off boredom. When she is just being the Mary who is not being inappropriate or working an agenda, she is a decent sort of person. Unfortunately, that decency seems but a small measure of who she is as a whole.