Marfa Petrovna Svidrigaïlov
Is Marfa a victim? Or a victimizer? Or both?
Dang you, Dostoevsky, for making things so complex.
To be fair, it's hard to say because we never meet her. We know that she paid Svidrigaïlov's debt to get him out of prison and used her money to keep him from being prosecuted for abusing the young girl he drove to suicide. Nice lady.
She also encourages him to extort sexual favors from their servants. She saves Dounia's reputation in the community (though she ruined it in the first place) but in the meantime hooks her up with her relative, Pyotr Petrovitch Luzhin.
She doesn't have the best taste in men (she's in good company with the rest of the women in this novel...not that they have much of a choice), and she probably thought she was doing a good deed...even if she just wanted to keep Dounia away from her man.
Additionally, her ghost seems to be harassing Svidrigaïlov, who definitely had something to do with her death.