K is for Harmony Kendall
Harmony Kendall was a fellow student at Sunnydale High. She was a friend and crony of Cordelia Chase, taking over as "Queen Bee" when Cordelia teamed up with Buffy and friends. Harmony was bitten by a vampire on Graduation Day at the end of season 3.
Harmony reappeared in season 4 as a vampire, and in a relationship with Spike. As a vampire, she flitted between Buffy and Angel, adding some comic relief as she tried her best to be evil. Harmony wound up as Angel's secretary once he had taken over Wolfram & Hart.
Harmony was fun. She did well as Cordelia's bitchy side-kick and Queen-Bee, adding the occasional snide comment, but it was as a vampire that she really came into her own. Harmony was selfish and self-obsessed, and ditsy as anything. She couldn't make a plan to save her un-life. She had Cordelia and Anya's habit of saying what she thought, usually to the detriment of Spike's (or her own) scheme. She was terrible at being evil and, without a soul, she couldn't be good either. Harmony added comic relief to every situation she was in, but there were also moments of sympathy for her, particularly given her rather abusive relationship with Spike. Yes she was a vampire, but no one deserves a relationship like that.
My favourite Harmony moment though was the bitch-slap hair-pulling fight with Xander.