Harmony was a friend of Cordelia's in high school for many years, until she was turned into a Vampire on Graduation Night. A few months later she became involved in a romance with Spike, when he returned to Sunnydale to search for The Gem of Amara. She left him though after he tried to kill her.
She continues to live in Sunnydale, and has occasional minor run-ins with The Slayer.
Harmony has retained a lot of her human personality, except she is a lot more willing to kill. She's pretty much the same as her human self, just soulless. She doesn't seem to be able to fight very well, though she does have some Vampire strength. She's still probably the weakest vampire shown.