Since the primary characters in Pride and Prejudice are members of the Bennet family, Bennet would actually be a good word for the letter B. In fact, the novel is all about self-discovery. Elizabeth, as well as her father, realize just how improper the behavior of the Bennet family is and what the consequences of impropriety is. Elizabeth especially recognizes her father's blame for the actions of his family since he laughs at their folly rather than attempts to correct any behavior.
A good word choice for the letter U could be unwed. Lydia was guilty of running off with a man she was unwed to. Not only that, when Darcy spoke with Wickham, Wickham made it quite clear that he actually had no intention of marrying Lydia. We learn this in Mrs. Gardiner's letter to Elizabeth explaining Darcy's involvement in Lydia and Wickham's marriage. Mrs. Gardiner explains that Darcy learned from Lydia that she fully believed she and Wickham would be married and that to her "it did not signify when." Hence, Darcy understood that the only thing to be done was to "secure and expedite the marriage, which, in [Darcy's] very first conversation with Wickham, [Darcy] easily learned had never been [Wickham's] design" (Ch. 52).
The word zesty could be a good word beginning with Z to describe Elizabeth. Zesty means to be full of energy and active. We certainly do get a glimpse of Elizabeth's zest when she decides to walk three miles from Longbourn to Netherfield, immediately after a rain, in order to be with her ill sister. In fact, Austen describes Elizabeth as "crossing field after field at a quick pace, jumping over stiles and springing over puddles with impatient activity" (Ch. 7). Hence, we see that this description of Elizabeth proves her to be very zesty and very active indeed.
L-Longbourn. (The Bennet's home)
N-Netherfield. (Bingley's home)
U-Uncle Gardiner (Bennet's uncle)
X-eXpected (Marriage was an expectation)
Y- Mrs. Younge (Helped to hide Wickham and Lydia while their in London.)
Z-Zealous (Characterization of Mrs. Bennet regarding the marriage of her girls.)