Maria Bertram is not a nice girl. She's arrogant and rude and is often mean to her family. She's also selfish and greedy – she wants to marry for money just so she can escape Mansfield Park and her controlling father and lead a fun and fabulous lifestyle somewhere else.
However, even though we hear about Maria's bad behavior, and see her flirtation with Henry and her rudeness towards Mr. Rushworth, we often don't hear Maria speak. Like the other Bertrams, Maria has learned to be a good actress around people like her father and her aunt. It's part of the reason why Sir Thomas is as shocked as he is by his daughters' behavior at the end of the novel.
Maria is almost doubly doomed, though, because she is also Mrs. Norris's favorite. If Sir Thomas taught Maria how to pretend really well, then Mrs. Norris taught her to think no wrong of herself and to have an out-of-control ego. However, it's obvious that Maria feels contempt towards Mrs. Norris, and she usually puts her acting skills to use to hide that sentiment from her admiring aunt. But not always:
Maria was just discontented enough to say directly, "I think you have done pretty well yourself, ma'am. Your lap seems full of good things, and here is a basket of something between us which has been knocking my elbow unmercifully." (10.53)
So Maria is a rebellious and "discontented" daughter and niece who hides her true feelings out of necessity. But she's also a woman who falls in love with Henry Crawford, a man who manipulates her, uses her, and doesn't love her back. And, aside from being a figure of sympathy, Maria plays an important role in the novel. She demonstrates a lot of themes about gender and gives us a glimpse into the restrictions women had to deal with in this time period, as well as the unfair way women were blamed for scandals more than men were. The narrator explains,
He [Henry Crawford] was released from the engagement, to be mortified and unhappy till some other pretty girl could attract him into matrimony again [...] while she must withdraw with infinitely stronger feelings to a retirement and reproach which could allow no second spring of hope or character. (48.12)
Maria is the fallen woman in the end, shunned by society and living with a broken heart and a ruined reputation. Within the book she's an important figure of social and political commentary, as well as a character who stirs up a lot of trouble.