Clytemnestra "Clytie" Sutpen
Clytie doesn't seem like too much of a player for most of the book, but in the end, she's the one who – spoiler alert! – takes down Sutpen's Hundred. For good.
This woman is Henry and Judith's half-sister. Her father, Sutpen, brought her as a slave to Jefferson from Haiti. Though she is black, Clytie has many of the advantages of a white person at the time: after all, she lives the big house with Judith throughout the war. In fact, she has been living at Sutpen's Hundred longer than anyone and, by the end of the story, she is arguably one of the most powerful members of the household. No one gets in unless she says so – including Wash Jones (who is white). Well, unless they push her down, like Rosa does.
That's a lot of power, considering how the characters generally treat the non-whites around them. So what gives? Why is Sutpen okay with all this? Well, it seems like he acknowledges Clytie as his child because, as a daughter, she doesn't threaten his dynastic plans. And you know Sutpen and his dynastic plans.
Maybe because of the privileges she's been given, Clytie has a strange sense of loyalty to the Sutpen household. She is sympathetic in her desire to give Charles Etienne a decent life and in her selfless (and suicidal) decision to burn Sutpen's mansion to the ground in order to protect Henry from the law. Wow, Clytie, we didn't give you enough credit.