Add some stuff on her background (bad upbringing) and morals.
Mary Crawford really seems like she should be the heroine of this book. She's charming and funny and witty. She proves the old adage of "opposites attract" when she falls for Edmund. She's tied in to a number of the book's major themes, including ideas on communication and on activity vs. passivity. So what is she doing playing second fiddle to the book's actual heroine, Fanny?
Well, the fact that Mary isn't the heroine may be precisely the point. Mary has a lot of typical heroine traits going for her. She even has a lot in common with her witty creator, Jane Austen, and with a fellow witty and spunky Austen heroine, Elizabeth Bennett. But in a book that has an awful lot to do with assumptions (especially false ones), appearances, and misunderstandings, Mary – who is not the heroine – has a very important role. Instead of being the heroine readers assume she could be and maybe even should be, Mary is cast as the foil and the occasional, though unknowing on her part, nemesis to Fanny.
As a result, Mary's presence in the novel disrupts our preconceptions and expectations. She challenges the typical roles we assign to characters as well. You may have even occasionally found yourself rooting for her at Fanny's (our protagonist's) expense.But as a character existing in what is essentially Fanny's story, she becomes rather ironic as our almost – but not quite – heroine. Mary foreshadows her own story at the start of the book: "Oh dear – let him stand his chance and be taken in. It will do just as well. Everybody is taken in at some period or other" (5.18).
In a way, Mary was then "taken in" by Edmund and rather proved herself right when she noted that marriage, and romance, are a gamble. After Edmund un-ceremoniously dumps Mary when he deems her too morally lacking, Mary "was astonished, exceedingly astonished – more than astonished. [...] She tried to speak carelessly; but she was not so careless as she wanted to appear" (47.30).
Speaking of "morally lacking," what are Mary's morals? What is about her that Edmund and Fanny so strongly object to? Well, we know that Mary objects to Edmund becoming a clergyman. But Edmund expects that he can eventually convince her to marry a "poor" clergyman, despite her desire for a life of comfort and her general dislike of religion. However, Edmund can't get past her reaction to Maria and Henry's affair. Rather than being horrified by Henry and Maria's behavior, she considers it mere "folly." She's most upset that Henry was stupid enough to get caught, not that he was committing adultery with Maria.
Though Mary is so likable in so many ways, most people really wouldn't agree with her lax attitude toward her brother's behavior. So where does her sense of right and wrong come from? This ties into the "nature vs. nurture" question, which is raised repeatedly throughout the novel. In other words, was Mary simply born with the personality and principles that she has, or did they develop as a result of her life circumstances?
Though we don't know much about her past and upbringing, we do know that she and Henry were, at least in part, raise by their uncle, Admiral Crawford. When we meet Mary, she has left her uncle's house. As the narrator informs us, "Admiral Crawford was a man of vicious conduct, who chose, instead of retaining his niece to bring his mistress under his own roof" (4.15). We only get little, suggestive snippets of information like this, which may lead us to conclude that Mary wasn't raised in the healthiest family environment. What do you think? Can you find any hints about Mary's background or upbringing that shed light on her opinions and morals?
Mary took a risk and it didn't pay off. But the fact that she took the risk in the first place is the sharply contrast between her and Fanny. Mary declares throughout the book that she prefers doing things and being active and taking risks: "I am not born to sit still and do nothing. If I lose the game it shall not be from not striving for it" (25.25).
Mary does end up losing Edmund in the end, but it was doubtful, or at left deliberately vague, as to whether not Mary and Edmund would have made a good couple anyway. In retrospect, they may have kept hesitating and dancing around each other for some very good reasons. And, ultimately, Mary may have performed the role she was supposed to: she contrasted to Fanny and cast Fanny in a light that was eventually appealing to Edmund. As our almost-heroine, Mary effectively made sure that the "actual" heroine ends up with her hero.