A kind, sweet, quiet young woman, the oldest Bennet sister is Elizabeth's best friend and is in love with Charles Bingley.
Jane probably wins "Most Saintly" in the Pride and Prejudice yearbook. She never judges anyone, always gives everyone the benefit of the doubt (even the horrible Caroline Bingley!), and generally acts just as a proper young woman should. Ordinarily, this would make her the big winner, since nineteenth-century novels tend to want their young women to be exactly like this. Here, instead, Jane's behavior actually almost causes her to lose everything. Why? Well, we'll throw a couple of possibilities out there:
Possibility #1: Austen isn't too keen on the good-quiet-understanding model of female behavior. Jane is nice and all, but imagine if she were the main character instead of Elizabeth – total snoozeville. Not only that, but it turns out that there is such a thing as being too demure. Jane is so shy with showing Bingley how much she likes him that no one around them can tell whether she's really into him or not. While flinging yourself at guys Lydia-style isn't the way to go, Jane is way too far to the other side of the spectrum.
Possibility #2: Jane is in the novel to give Elizabeth one more thing to be wrong about – and to have to figure out from another person's perspective. Elizabeth is really committed to her own point of view in the beginning, and part of her deal is trying to see situations the way other people see them – you know, empathy. Think about when Charlotte points out that Jane isn't sending out enough signals to Bingley, and Elizabeth gets all huffy and says that anyone who knows Jane well would know she loves him. Of course, not everyone knows Jane as well as Elizabeth does, so this is a pretty flimsy argument. When she reads Darcy's letter and finds out that he totally couldn't tell whether Jane loved Bingley or not, Elizabeth has to go back, reexamine, and learn.Jane Bennet Timeline