Juliet Landau starred as Spike's season two paramour Drusilla. This waifish vampire is insane (literally), driven mad by the infamous Angelus. Though it seemed she should hate the vampire who killed her family and sired her, she didn't, and not only that - she seemed to choose him over Spike! Dru was last seen unconscious in the arms of Spike as he drove out of Sunnydale. Per the third season, she gave Spike the boot (I knew that one was coming). After that, she was only seen in cameos in the remembrances of Spike and Angel.
Spike and Dru did make an insanely cute couple, but he also hooked himself up with Harmony (former Cordelia fanclub member turned vampire on "Graduation Day") and Buffy.
Then, there's Angel (in his good persona) / Angelus (in his evil persona). David Boreanaz had free reign to explore the vulnerable, angst-ridden Angel, in love with the one person sworn to kill vampires like himself (and that would be Buffy), and to play the tortuous, evil Angelus, hell-bent on destroying the one that Angel loves, and trying to pull Dru away from Spike. Angel was sent to hell with soul newly restored at the end of the 2nd season, but returned fairly early in the 3rd season, still on the good side. However, things have grown too complicated between him and Buffy and he bailed out and moved to Los Angeles where hel started his own business, Angel Investigations. For more updates on Angel, you'll need to check my Angel page..
Of course, it wouldn't be right to not show something of the force that fights these vampires. Buffy (played by Sarah Michelle Gellar) is coached by her watcher Rupert Giles (played by Anthony Stewart Head) or Wesley (depending on what season you're watching). She is aided in her fight against the vamps by her friends Willow (played by Alyson Hannigan), Xander (Nicholas Brendon). The cast also started out with Cordelia (Charisma Carpenter), but she left at the end of season 3 and joined Angel in L.A. Angel was also considered one of the Scooby Gang/Slayerettes, depending on the episode and season. Joining in season 2 and departing in season 4 was Oz (Seth Green) who transformed into a werewolf with the full moon. The gang also grew additional family in the form of Buffy's kid sister, Dawn (Michelle Trachtenberg), who was really a key that unlocked the dimensions and was turned into a human for Buffy to protect.
As Buffy died briefly in season 1, a new Slayer was called and the show was graced with Kendra (Bianca Lawson) who was sadly killed in season two. Her death called another Slayer, bad-girl Faith (Eliza Dushku) who turned very bad in season 3, but fought for the good guys at the end. Pictures below.
Of course, there have been episodes where some of the bad guys play good guys, and some of the good guys go bad. Here is a picture of Xander and Willow in the episode "The Wish" and Evil Willow in the sixth season.