A complex character is a character that is multi-faceted, showing different levels of character development throughout a work. Both Elizabeth Bennett and Mr. Darcy met this description.
Elizabeth is both playful and reserved, well-mannered and intelligent. Her wit is seen throughout the novel. She is proud and makes hasty judgments, and then has to work to set them right. Elizabeth is very aware of her sister's desires and emotions, but is blind to her own for the majority of the novel. She is not wealthy or a part of high society, yet she can easily fit in with people of this class.
Mr. Darcy is very reserved, almost to a fault. He is also proud and makes hasty judgments which he later regrets. Mr. Darcy is a loyal friend, but mistakes his role with Bingley, becoming more of a manipulator than a friend. While he cares deeply for those he is close to, it is incredible difficult to get close to him, because he sees his class as far superior to those around him. Yet, though he stops Bingley from pursuing Jane, he cannot stop himself from pursuing Elizabeth, showing a complexity and duality to his nature.
Both characters are proud, and both characters are prejudiced, two vices that nearly keep them apart. Though the traits are seen in both, Elizabeth is more the "prejudiced" one and Darcy is more the "prideful" one. Through these characters, Jane Austen is able to show the folly in both traits.