Anya

Ex-Vengeance demon, and ex patron Saint of scorned women.

Anya first appears in the episode 'the Wish' as Anyanka (the demon). She loses her powers thanks to the Scooby Gang.
Anya is continually trying to regain her powers during Season 3, leading to her telling a vamp to kill Willow when she figures out it isn't vampy doopelgangland Will.
At the end of Season 3, Anya asks Xander to the Prom, cos he's less uuky than the other guys. They go together, but don't seem to get on all that well; Xander gets bored as all she does is talk about things she has done to men in the past (revenged them).
Anya leaves before the Mayor's Ascension-she's lived through one before-not good.
During Season 4, Anya returns, intent on seducing Xander because she can't stop thinking about him, and she feels the solution is to sleep with him so she can forget about him. This plan backfires, and Anya and Xander become a couple.
At the end of Season 5, Xander proposes to Anya-she accepts, but only on the terms that he ask her again 'after the World doesn't end.'
At the start of Season 6, Anya is desperate for Xander and her to make an announcement about their engagement. Xander keeps putting it off.
Anya is absolutely terrified of rabbits.
Emma Caulfield plays Anya Emerson.