Lydia is a complete flirt with military men stationed in Meryton. She also seems to be a bad influence on her sister Kitty to do the same thing. Lydia gives the whole Bennet family a bad reputation. Elizabeth comments on this:
"If you were aware," said Elizabeth, "of the very great disadvantage to us all, which must arise from the public notice of Lydia's unguarded and imprudent manner; nay, which has already arisen from it, I am sure you would judge differently in the affair.
On top of that, Lydia elopes with Wickham without a proper marriage and her family knowing.