I sort of covered starring NPCs here and here, but never mind.
He had many faces; he wore many hats.
Sir Derek Jacobi on Sir Laurence Olivier
I can expand on the Sibyl a bit. An enemy who perceives that most vampires don’t deserve immortality - on which I agree, but usually it’s the PCs that feel this way and do something about it.
I played up the “mad seer” angle - she read signs in her own blood drawn from the pricking of her thumbs and from animal entrails, as she fed from animals (while nobody was looking) - so the PCs missed the significance when she explained her views early on, that vampires “are a plague on the Earth, something that should not be,” and were also caught off-guard and failed to connect the dots when she turned up in Elysium (where there was a murder behind the scenes that they had to help cover up and then investigate) acting quite normal...
More on Walker is still a spoiler. Shh.
For published, I go back quite a way, to Sheriff in Chicago in Masquerade. An entirely understandable traitor to his whole clan, a bully from a position of safety, a fraud... and still pretty damn dangerous. Sheriff went on to become a job in traditional Masquerade, Dark Ages and Requiem cities, the seemingly random title Eric got stuck with in True Blood, and generally a mark that a character was Bad News.