Though Elizabeth is shocked to learn of Collins and Charlotte becoming engaged, Austen foreshadows this turn of events in an earlier scene. When Elizabeth and Charlotte are discussing Jane and Bingley’s courtship early in the novel, Charlotte makes it clear that “securing” a partner is the most important thing, implying that she herself is uninterested in getting to know the “disposition” of her potential husband until after they are married, saying, "When she is secure of him, there will be more leisure for falling in love as much as she chooses." She later goes on to say:
"Happiness in marriage is entirely a matter of chance. If the dispositions of the parties are ever so well known to each other or ever so similar beforehand, it does not advance their felicity in the least.”
This is precisely what Charlotte does when it comes to Collins—she has only just met him and knows very little about him, aside from the fact that Elizabeth has turned down his proposal and considers him to have an awkward and condescending character. In that way, Austen subtly foreshadows Charlotte’s impromptu betrothal.
This moment also establishes Charlotte as a foil to Elizabeth—whereas Elizabeth is discerning about who she will marry and chooses to wait until she finds someone with whom she falls in love, Charlotte is content to marry someone she does not love in order to have financial security.
Throughout the novel, the youngest Bennet sisters—Mary, Kitty, and Lydia—act as foils to the oldest daughters, Jane and Elizabeth. While Jane and Elizabeth are caring, mature, and know how to behave with proper manners, the three younger Bennet sisters display signs of naivety and an overall lack of social awareness. Specifically, Kitty and Lydia engage in over-the-top flirting, and Mary—who is more of a recluse—insists on publicly playing the piano very poorly.
In his letter to Elizabeth explaining why he encouraged Bingley to let go of his feelings for Jane, Darcy names these differences between the two sets of Bennet sisters:
“The situation of your mother’s family, though objectionable, was nothing in comparison to that total want of propriety so frequently, so almost uniformly betrayed by herself, by your three younger sisters, and occasionally even by your father […] let it give you consolation to consider that, to have conducted yourselves so as to avoid any share of the like censure, is praise no less generally bestowed on you and your elder sister, than it is honourable to the sense and disposition of both."
Darcy directly states that Jane and Elizabeth's dispositions are worthy of praise, while their younger sisters have displayed a "total want of propriety," or a lack of manners. The differences in temperament and behavior between the eldest and youngest Bennet sisters highlights how neither socioeconomic class nor family can be considered the ultimate determinants of behavior.
Throughout Pride and Prejudice, Wickham acts as a foil to Darcy, since his presence often brings out certain vital pieces of information about Darcy’s character. Specifically, Wickham is Darcy’s foil in that he appears to be a kind, affable man while Darcy appears to be arrogant and standoffish. In reality, however, Darcy is kind and considerate, while Wickham turns out to be a liar who willfully manipulates young women into giving him what he wants (whether companionship, their inheritance, or both).
This quote from Elizabeth (while conversing with Jane) captures the ways that, in comparing Wickham to Darcy, she realizes how wrong her judgments of the two men had initially been:
“There certainly was some great mismanagement in the education of those two young men. One has got all the goodness, and the other all the appearance of it.”
This moment also shows how Elizabeth’s pride and prejudice got in the way of her being able to see both Darcy and Wickham for who they really are. She considered Darcy to be wealthy and arrogant and Wickham to be a hapless victim of the other’s pride. Here, she learns that she cannot make such judgments based on first impressions, rumors, or a given person's socioeconomic class.