Written by Zinna
Faith can first be seen slaying a vampire behind the Bronze, much to the surprise of Buffy and the Scooby Gang in the episode entitled "Faith, Hope, and Trick" (3rd episode, Season 3). Faith appeared to be called to Sunnydale as a replacement for Kendra, Drusilla's first slayer kill (Becoming Part 1). Later, Faith reveals she is fleeing the ancient, cloven-hoved vampire that killed her watcher. In the same episode, Buffy helps Faith face the vamp. Faith is able to best her villain by plunging big wooden telephone-pole-sized stick into the villain's heart.
Faith takes the position as Buffy's co-slayer. Initially, she is well received by the Scooby-Gang, especially Xander, who can be seen drooling over her tales of wrestling an alligator while naked. Buffy as well, appears to welcome the assistance, grateful to have someone to share her burden. After a few patrols, Buffy is a bit turned off by Faith's enthusiastic slaying attitude. Faith appears to get-off on the kill, at times she admits to being aroused by it. Buffy, having slayed the love of her life, Angel (Becoming Part 2), has a hard time relating to Faith's point of view. It would also appear that Buffy is annoyed to be sharing her friends, her home, and even her fries with Faith. Faith, on the other hand, has no problem helping herself to Buffy's amenities. In time, however, the two slayers bond becoming Sunnydale's chosen two.
Throughout the first 11 episodes of season 3 we can find a sexually charged Faith slaying vamps along side Buffy, battling evil uglies with the gang, and hastily snatching virginity from a very willing Xander (The Zeppo), much to his surprise and ours. Besides seeing Faith fearful to face her watcher's killer (Faith, Hope, and Trick), Faith appears to have nerves of steel and an attitude to match. She's an ultimate bad-ass.
Over time, Faith and Buffy's dynamic slides in new directions. Though the details are never spelled out for us, it is clear that Faith comes from nasty beginnings. Mostly, we know her mother is dead and when her mother was alive, she was an alcoholic. Other than Buffy and the Scoobies, there doesn’t seem to be anybody in the world that cares about Faith’s welfare. Faith begins to envy Buffy, but she never lets on (until Enemies when her jealously is exposed). Instead, she tries to convince Buffy that slaying is the ultimate rush and consequently, the ultimate power, making them above the law.
In "Bad Girls", Faith leads Buffy on a reckless slaying joyride that ends in tragedy. Still high on a rush from torching a vamp nest during school hours, Buffy and Faith comb the back allies knocking vampires off one by one until they mistake the mayor's assistant for one of these evil minions. Buffy does not realize their mistake before Faith accidentally stabs him in the heart. After several attempts to dodge responsibility, Faith finds her deed to be irreconcilable, and concludes that she is, in fact, no different from the bad guys. At this point Faith joins forces with the Mayor and becomes one of the hardest villains the Scoobies have had to deal with since Angel lost his soul.
In the third season finale, Faith pierces Angel with a poisonous arrow, rendering him helpless to a very painful, very fatal illness, and the only cure is the blood of a slayer. Buffy, unwilling to lose Angel again, goes off in a fury and red leather pants, ready to kill Faith and bring her body for Angel to feed on. Buffy succeeds in stabbing Faith, but fails to take her life. In her last breaths Faith jumps from the roof of their battle to moving truck below. Later, the mayor finds Faith in coma that doctors believe she will never wake from. But we haven't seen the last of her yet.
Faith awakes in season 4 to a whole new world without the mayor - she feels completely alone. Her first priority is to get even with her favorite slayer and that she does by taking Buffy identity. Being Buffy sends Faith for a loop as she realizes having loved ones and being loved comes with responsibility. Before Buffy is able to switch back their bodies, Faith feels obligated to resume Buffy's duties as a slayer, something she nor we ever thought she would feel again. This is the beauty of Faith's complexity. Here, we can see Faith's evil stems from self-loathing not bloodlust. Of course, she thinks cannot be forgiven, so she continues to run once she is returned to her rightful body. This is the last we see of Faith on Buffy the Vampire Slayer, but don’t think that it is the last of her (She appears in episodes of Angel soon after her departure from Sunnydale, of course mayhem ensues).