When a Slayer dies, one potential Slayer becomes The Slayer. There isn't supposed to be more than one Slayer at any given moment. However, since Buffy died and resurrected, a second Slayer came into play. Kendra coexisted with Buffy and when she passed away, Faith became a Slayer.

One day Faith just walked into Sunnydale and met with Buffy and friends. She began acting as a companion and aide in Buffy's nightly rounds. Faith was a good fighter but had a much more careless and aggressive style than her colleague. In fact, Faith confessed that she would get thrills out of hunting and killing vampires. She also enjoyed a worry free lifestyle, partying all night, indulging into any need and desire that she might have. Soon the two Slayers clashed as Buffy was worried about Faith's lack of seriousness and commitment about their sacred calling. She was also a tad jealous about her counterpart's vibrant and magnetic personality which seemed to attract both Willow and Xander.

Faith had a one night stand with Xander but her nature forbade her from committing to the guy who was very much into her. Willow heard of the matter and since she herself was keen on Mister Harris, she was hurt. Facing rejection from the Scooby Gang, Faith found herself drawn towards Mayor Richard Wilkins who quickly became sort of a father figure to the family-less Slayer.

Acting on her own free will, Faith turned into an enemy of the Scooby Gang, helping the Mayor in his treacherous plot to summon an ancient and pure demon to reclaim Sunnydale for the demons. She is relentless and goes to the extent of poisoning Angel which leads to a battle between the Slayers. Buffy comes out victorious and Faith ends up in a coma.

Faith remains for several months in this unconscious state but finally recovers. She blames Buffy for her fall and the loss of the only person in her life for which she cared: Mayor Wilkins. She uses a mystical device to have her vengeance upon the Slayer. The device makes them change bodies which enables Faith to escape from the grasp of the Council while Buffy gets captured. Faith tries to pose as Buffy but the original one comes back and convinces Giles and her friends of her true nature. Willow and Tara help Buffy regain possession of her body and Faith finally escapes to Los Angeles.

Once in L.A., she is hired by the evil lawfirm Wolfram and Hart to kill Angel. She attacks him and his team, eventually kidnapping and brutally torturing Wesley, her former Watcher. When Angel shows up to rescue Wesley and the fight with Faith ensues, she breaks down and demands he kill her because she's bad. Angel then takes it upon himself to save Faith from herself. But both Buffy and the Watcher's council show up, looking for revenge. Faith chooses to accept the consequences of her past actions and turns herself into the police.

Not much is known about Faith until Buffy is getting ready to fight The First. Faith comes back and tells Buffy that she's now on her side and willing to do what it takes to stop this new and powerful threat. Buffy is dealing with a lot of pressure and her actions seem to alienate the potential Slayers who are working under her leadership and care. After a while, the potentials rebel against Buffy and they instate Faith as their new leader. This, however, is short-lived; Faith, who has always been far more careless than Buffy, leads the potentials into a deadly trap which they only escape thanks to the intervention of Buffy herself. Buffy takes again the role of Alpha dog and Faith doesn’t really seem to mind.

In the final battle against the First, Faith is part of the army of Slayers that fights the elder vampires. She helps Buffy's cause to the very end and then she disappears again.