Chapter 39 certainly illustrates Lydia's character quite clearly:

"And we mean to treat you all," added Lydia; "but you must lend us the money, for we have just spent ours at the shop out there."

"Look here, I have bought this bonnet. I do not think it is very pretty; but I thought I might as well buy it as not."

"And when her sisters abused it as ugly, she added, with perfect unconcern, 'Oh! but there were two or three much uglier in the shop...'"

"...and the waiter was told that he need not stay. Lydia laughed, and said -- 'Ay, that is just like your formality and discretion...But he is an ugly fellow! I am glad he is gone. I never saw such a long chin in my life.'"

"...I will answer for it, he never cared three straws about her -- who could about such a nasty little freckled thing?"

"How nicely we are crammed in!" cried Lydia. "I am glad I bought my bonnet, if it is only for the fun of having another bandbox!

"She says Lizzy had better have taken Mr. Collins; but I do not think there would have been any fun in it."

"...we had such a good piece of fun the other day at Colonel Forster's...We dressed up Chamberlayne in woman's clothes on purpose to pass for a lady -- only think what fun!...except my aunt, for we were forced to borrow one of her gowns...Lord! how I laughed! and so did Mrs. Forster."

"Oh! Mary," said she, "I wish you had gone with us, for we had such fun! As we went along, Kitty and me drew up all the blinds, and pretended there was nobody in the coach...And then, when we came away, it was such fun!...I was ready to die of laughter..."

"But of this answer [Mary's] Lydia heard not a word. She seldom listened to anybody for more than half a minute, and never attended to Mary at all."