A poor relation taken in by the Rostovs, Sonya has a brief romance with Nikolai but then falls into a thankless caretaker role.
You know who Sonya reminds us of? Basically every single Dickens heroine there ever was. Seriously, Dickens – who was one of Tolstoy's favorite novelists – created tons of these totally self-sacrificing, humble, perfectly angelic young women. (Think Esther in Bleak House, or Amy Dorrit in Little Dorrit, for example).
For Dickens, this is the female ideal, and all his caretaker girls tend to be rewarded with marriage and some kind of happy ending. But in War and Peace, it does no one any good to completely give in to the whims and desires of others. By the end, Sonya is basically just a piece of the furniture in Nikolai Rostov's house, reduced to living with the family of the man who promised to marry her (but didn't) and taking care of his children. Yikes. If that isn't a warning, we don't know what is.