Fanny was born to a "lesser" branch of the Ward family. The eldest sister married a baron. The next married a well-to-do parson. The third married for love and gave up riches. Some years later, the two older sisters—well, mostly the middle one, Mrs. Norris—concocted a scheme to help their sister who was overburdened with children. They decided to raise one for her and chose Fanny. At age 10, Fanny goes to live with her Bertram cousins to learn how to be a gentlewoman. Tellingly, Mrs. Norris harangues poor, young Fanny on the drive from Portsmouth to Mansfield about how grateful she must be to her relations. Any sign of disappointment or sadness or anger will be interpreted as ingratitude and the punishments will be terrible. Consequently, Fanny learns to bend to the wishes and wants of everyone around her. She lives in a nebulous area on the social spectrum. She's not treated as part of the family, but she's not a servant or friend either.
A good portion of the first third of Mansfield Park actually focuses on everyone around Fanny. We learn about her silly female cousins and her serious cousin, Edmund. We learn about Sir Thomas and Lady Bertram. Austen gives us snarky descriptions of Mrs. Norris, who is always happy to volunteer other people's time and resources but ducks out as soon as someone suggests she take an active role. She's an awful person, but a very entertaining character. When we actually meet Fanny, it's hard not to feel sorry for her. She's disregarded and shunted aside by everyone but Edmund. It's no wonder that she grows to love him. I said before that she's firm in her principles. This is true, but she seems to get away with declining to participate in a highly inappropriate play or give Henry Crawford any romantic encouragement by coming so close to actually dying of embarrassment that the other characters take pity on her. Fanny gets happiness at the end of Mansfield Park, though it seems mostly to come about after the other characters have wised up to their follies and flaws.
I also said before that this might be Austen's truest novel. What I mean by that is, I had no problem believing the events of Mansfield Park. That there are so many wildly happy marriages in the other novels and so little real world consequence of people's follies (excepting the Wickhams), that it's hard to really believe them. As I read Mansfield Park, I wonder how many women (possibly Austen herself) the author met who were similarly ignored and emotionally battered by people with more power. I was strongly reminded of Villette, by Charlotte Brontë. In that novel, too, a female character on the sidelines watches life pass before her. Mansfield Park is a sad story, even if it ends with marriage and happiness.