Dounia is strong and steady, young and beautiful, intelligent and educated. Yet, she isn't the best judge of character. She learns over the course of the novel, though. In some ways Dounia is a device used to drive the plot forward. Her relationships with Luzhin and Svidrigaïlov create most of the novel's subplots and contribute to Raskolnikov's decision to murder the pawnbroker.
In other ways she is much more than a plot device. She represents a less depressing view of Russia and the Russian people than we often get in the novel. She grows as a character in a less ambiguous way. Instead of being in a relationship to save someone (Svidrigaïlov), or to escape poverty (Luzhin), she finds a genuine friend, companion, and husband in Razumihin, a man who loves her brother as much as she does and who understands what she's gone through.