Reputation
Clarissa's family is nouveau riche. They have the money but not the reputation to be influential, so they place enormous pressure upon their children to comport themselves well. When Clarissa engages in a secret romance with Lovelace -- a man of low moral fiber, -- her parents all but disown her. She is forced to leave their home and is financially cut off. Although she remains pure, she is rejected by the whole family and left to her own devices. Her parents' only concern is the dishonor she has brought to her family by consorting with a scoundrel like Lovelace who has already embarrassed her sister by leading her on in a romance. They are not so much worried about Clarissa's safety as her rebellion. Appearances are what matter in this story.
Purity
Clarissa has grown up believing in the moral command of purity. She had values instilled from a young age, still doubtless concerned with outward impression more than actual integrity. She isn't like the rest of her family, however, because she truly believes in these ideas. When Lovelace tempts her, she holds true to herself by maintaining her chastity. She doesn't even submit to his entreaties after he's raped her and robbed her of her virginity. After that incident Clarissa wastes away into illness, unable to live with this defiled version of herself. She makes preparations for her impending death with reservation and clarity. She forgives Lovelace for taking her purity from her, but she remains disgraced and succumbs to a pious death.
Manipulation
Lovelace is a pretty despicable guy. When he meets Clarissa while courting her sister, he immediately hatches a plot to have her for himself. Flattered by his advances, she engages in correspondence with him. He wants to conquer her sexually, however, and is not content with the socially prescribed boundaries of their relationship. Meanwhile Clarissa's family is conspiring against her and Lovelace by forcing an engagement with Roger Solmes. Clarissa despises this man and decides to run away with Lovelace in order to avoid the marriage. With Lovelace, Clarissa still remains adamantly chaste. This unattainable prize is too much for him, causing him to obsess over her. He moves her into a brothel, drugs her one night, and rapes her in the hopes of finally winning her over. Still, Clarissa awakens and holds even tighter to her virtue, although she stops eating because she's so disgusted by the horrible violation. At first Lovelace had no intentions of marrying her, but now considers it for the sake of finally making Clarissa his own. She refuses even that, though, having learned firsthand of his vileness.
