Faith

Called after Kendra's death, Faith is a pretty wild Slayer. Like Buffy she doesn't play by the rules. He Watcher killed and hunted by a vampire, Faith winds up in Sundyale where she meets the gang. Feeling a little left out - and considered the 'second best - Faith starts to act crazy and eventually kills a man by accident. Far from being sorry, she turns evil and fights against Buffy. Buffy attacks her, stabs her and pushes her off a building, sending her to hospital in a coma.
After eight months, Faith awakens and seeks revenge. She uses magic to switch bodies with buffy and wreaks havoc in her life. Back in her own body, she flees to L.A. and tries to get Angel to kill her. He manages to reason her and she turns herself in to the police, on the road to redemption.
Spending two years in jail, Faith escapes when someone tries to kill her and joins Buffy in Sunnydale to help her fight the First.