Drusilla

Drusilla is a major antagonist of Buffy the Vampire Slayer and one of the few vampires that have managed to battle Buffy and her crew over a prolonged period of time while living to menace them another day: no doubt she owes some of her survivability to her tendency towards staying in the shadows rather than engage in outright assaults, despite her madness she is a very clever and murderous woman who is capable of formulating many plans - often of surprising complexity or merit.

Drusilla is not, however, a stable villain - in fact she is psychotic even by the standards of vampires, this is down to Drusilla's tragic origin in which Angelus and Spike killed her family and tortured her to madness before finally making her into a vampire: surprisingly Drusilla doesn't show hostility towards the two (save for the reformed Angelus, now naming himself Angel) - in fact she seemed to have developed a twisted relationship with Spike, though it wouldn't last.

Drusilla is also shown to have several undefined psychic powers - which make her even more dangerous than a typical vampire and is also knowledgable on the dark side of life, it was by Drusilla's request that the powerful demon named The Judge was brought back to life as a "gift" for Buffy's birthday: though his claims of invincibility proved ultimately futile in a world that had advanced beyond the dark ages.