The role of Lydia in the moral pattern of the novel Pride and Prejudice, by Jane Austen, is to disrupt the natural order of things proposed by the society of Austen times, in which women were supposed to be sensible, balanced and, to a point, almost ornamental in terms of courtship and marriage.
Lydia disrupts the normality in the Bennet household by being a teenager with wild, romantic illusions, and every single intention of fulfilling them. She openly befriends male soldiers, flirts with them, and laughs out loud with them caring nothing for any form of public lady-like decorum.
To add to the scandal, she elopes with Mr. Wickham at barely 15 years of age, unmarried, and losing her honor. Her elopement stained the Bennet name as a whole, for they now would have to recognize that their daughter is "lose", "immoral", and "indecent."
Yet, the most unfortunate aspect of Lydia's behavior is that she does not regret it, and she learns nothing from her mistakes. Oblivious to all the heartache she has caused, she takes the role of precedence that women obtain after marrying only to brag about it. She puts her sisters down, asks Elizabeth whether she is jealous that it was she (Lydia) who ended up with Wickham, and she is blind to every indication by Wickham that their relationship was a brief caprice of which he is already tired.
Moreover, Lydia's marriage is an extension of her behavior: Both she and Wickham are spendthrifts who live above their means and ask their familyfor money. None of the sisters, Jane or Elizabeth, bothered to invite the Wickhams to their respective households. Eventually, it is said in the story that they were altogether ignored.
Therefore, Lydia's role is to take away the balance of the Bennet household, bring on chaos, and offer absolutely nothing to the present or future of the family.