Spoilers below!

Charlotte lucas is a very ambiguous figure in this book! After she marries Collins, she's happy, but he is not what is making her happy -the position and security make her happy. So when Mary comes to visit, Mary realizes that Mr. Collins is lonely and kind of sad, and he becomes attached to Mary, fairly naturally and innocently. The tension this causes between Mary and Charlotte highlights that Charlotte is indeed perceptive and a bit selfish to secure her own happiness, but also serves as a model of a woman working actively to create her own happiness. Collins gets a bit more fleshed out,and he's grown up a fair bit from the days of his first visit to the Bennetts. Mary ends up leaving her visit early to go to London (which is a much better fit for her) and Charlotte seems to take to heart that Collins needs some kindness from his wife even if its a marriage for convenience rather than love.

I really love Charlotte being kind of pragmatic and cold in this book,towards ends that suit her, and she doesn't excuse her choices. Even while she acknowledged that she did cut Mary out.