Charlotte Lucas is the intimate friend of the heroine Elizabeth (Ch.6). Jane Austen uses Charlotte to express one important aspect of the central theme of the novel, namely marriage.
Many women get married merely for the sake of getting married. This is a completely unromantic attitude to one of the most important decisions in one's life. In Ch. 6 Charlotte tells Elizabeth, "happiness in marriage is entirely a matter of chance." Charlotte gets married to Collins to escape poverty. In Ch.22 we read that her brothers "were relieved from their apprehension of Charlotte's dying an old maid" and that marriage for many women like Charlotte was "the pleasantest preservative from want." Towards the end of the chapter she tells Elizabeth, "I am not romantic, you know. I never was. I ask only a comfortable home."
Lady Catherine is the aunt of Darcy and the patroness of Collins. Throughout the novel Jane Austen attacks the snobbery of the aristocratic class through her. This can be seen especially when Elizabeth visits her at Rosings Park in Ch.29, "her ladyship with great condescension arose to receive them."
She serves as a foil to Elizabeth's character. Elizabeth's confrontations with Lady Catherine serve to highlight her independence, boldness and assertivenes, for instance in the same chapter Jane Austen tells us that whereas all the others were completely overawed by Lady Catherine and her stately home Elizabeth remains completely unfazed: "the mere stateliness of money and rank, she thought she could witness without trepidation."
Most significantly Lady Catherine serves as a catalyst to speed up Darcy's second proposal to Elizabeth. In Ch 58 Darcy tells Elizabeth that if she did not want to marry him she would have told Lady Catherine herself when she visited her at Longbourn in Ch 56. It is then that Elizabeth boldly tells her "you may ask questions which I shall not choose to answer" and hints to her that she will accept him if he proposes the second time,"and if I am that choice, why may not I accept him?" And when Lady Catherine asks her to promise not to get married to him, Elizabeth remarks,"I will make no promise of the kind."