DRUSILLA
A poor English girl blessed (or cursed) with the ability to see into the future, Drusilla was seen by the vampire Angelus as the personification of pure innocence. After torturing the piety out of her, as well as Drusilla's sanity, Angelus sired
her in 1860, following her admission into a convent.
Drusilla stayed with Angelus and Darla, until finally taking her own playmate -
Spike - in 1880. Spike worshipped Drusilla, and risked his life to save her when she was attacked in Prague in the late 20th century. He brought her to Sunnydale where he performed a ritual to restore her strength.
This was done, and Drusilla was restored to her 'normal' self - inhumanly
strong, totally insane, and psychically attuned to both present and future
events through her visions - but the whole ordeal left Spike in a wheel chair following a run in with the Slayer, and one nasty pipe organ. However, his wheel chair did not stop Spike from giving Drusilla one hell of a coming out party, assembling the body of the demonic beast known as the Judge. The dealings with the Judge eventually led to the removal of Angel's soul, reverting him back to Angelus. Promptly seeking out his Drusilla, Angelus began stealing away her affections for Spike.
Spike helped the Slayer stop Angelus from destroying the world in order to get Dru back, but afterwards, Dru wanted nothing to do with Spike.
She later reappeared in Los Angeles, where she sired her vampiric grandmother for the corrupt law firm Wolfram & Hart. Now in league with Darla, Drusilla caused havoc for Wolfram & Hart, before being set alight by Angel. Fleeing to Sunnydale to recover from the ordeal, Drusilla sought out Spike, only to
find him head over heels in love with the Slayer. Drusilla left Spike, declaring that he was so very lost.
That was the last we've seen of Drusilla, but she's presumed to be in LA
somewhere, with Darla.
(from 'Evil Incorporated' site)