Faith is another slayer, called when Kendra was killed.
Faith was always a "fly by the seat of her pants" type of gal. She was a no nonsense slayer. no apologies....want, take, have. A little messed up, she was still on the side of good. When she first came to Sunnydale she teamed up with Buffy and helped her slay the vampires. Lack of a discipline and a strange chain of events led Faith to become darker that she should have.
Faith, is the bad girl on the show. She turned bad when she killed a human, deputy mayor, who was not a good person anyways. She soon teamed up with the Mayor and didn't care who she killed, human or vampire. Buffy was on top of her list to be destroyed.
Buffy and Faith came head to head in Faith's apartment. Buffy stabbed Faith which made her fall off the building and land on a passing truck. Faith didn't die but went into a serious coma. She eventually woke up and was set on revenge. She managed to do a body switch with Buffy & tried to wreak havoc with Buffy's life as well as evade the police. After Buffy reclaimed her body, Faith fled to L.A> where she hunted Angel and tortured Wesley.
Faith is currently in prison for the murder of the deputy mayor