Sybil has basically one scene in the entire novel, but boy, is it intense. Drunken Sybil wants to be raped by a black man, and it somehow comes across as touchingly vulnerable. Like the narrator, our dominant emotion while reading the scene is sadness. The narrator and Sybil are two human beings together in bed who have somehow forgotten their humanity in the pursuit of their own ends. To Sybil, the narrator is essentially a domesticated black rapist there to carry out her fantasy. On the narrator's part, he had intended to use Sybil as a pawn in his quest to sabotage the Brotherhood. Fairly early in the night, however, the narrator realizes that she has no information and is interested in him for purely sexual purposes. He takes an almost protective stance towards her, pretending to acquiesce to her wishes while really coming to grips with the full extent of his invisibility.
We can also see Sybil as a product of society. The narrator hypothesizes that her attraction to the rape fantasy is simply the attraction to power. "They're taught to worship all types of power," he muses, and within that analysis we can see how invisibility becomes pervasive throughout society. Rather than being attracted to who the narrator is, Sybil is attracted to the myth of the black rapist that society has taught her.