A boy-crazy, totally un-self-conscious teenager (she's 15 when the story begins), the youngest Bennet sister runs off with Mr. Wickham, who is later forced to marry her.
We feel a little sympathy for Lydia, but it's hard. She's "untamed, unabashed, wild, noisy, and fearless" (51.4). She flirts with anything in a uniform; she spends all her money and makes her sisters lend her cash so she can buy them dinner; and she only stops talking about boys to look at a "very smart bonnet … or a really new muslin in a shop window" (15.7). She doesn't learn her lesson about Wickham, returning to Longbourn with the same "easy assurance" she had before she ran off with a total sleazeball (51.3).
But. The fact that a young female character written at the start of the 19th century is allowed to go off with some dude for a bunch of premarital sex and then isn't punished with death makes Pride and Prejudice about as girl-power as a novel written at this time can get. Yes, Lydia is impulsive and kind of moronic and generally a big drain on her family. But come on, the deck has kind of been stacked against her from the start. Lydia is only fifteen, and she's been raised by a woman who is exactly like her, and neglected by a man who couldn't care less.
One super important point to note: there was a whole etiquette about when women were allowed to "come out," meaning show themselves off in public as eligible for marriage. This was usually around the age of 17 or 18, and a lot of times a younger would have to wait until her older sister(s) were married—partly so they wouldn't be competing for the same dudes, and also partly because it was super expensive to dress a girl for a bunch of balls. (Lady Catherine actually criticizes the Bennets for not following this custom: "The younger ones out before the elder are married!" [29.34].)
But we learn that Mrs. Bennet "had brought [Lydia] into public at an early age" (9.36), meaning that she started going to balls too young, before she's learned how to behave in public or to tell when a guy is just trying to get you in bed. No wonder she gets seduced by the first good-looking smooth-talker who comes along.Lydia Bennet Timeline