In Slayer mythology, when one dies, the next is Chosen. At the end of the first series, Buffy was left for dead by the Master, and was in fact technically dead until being revived by CPR, courtesy of Xander. This activated Kendra, a Slayer who arrived in Sunnydale only to find her predecessor alive, and they then joined forces to fight Angelus, Spike and Dru, but Kendra was killed.

The death of Kendra activated Faith. When the evil vampire Kakistos murdered her Watcher in front of her, Faith escaped to Sunnydale to look up the other Slayer.  Arriving in Sunnydale in "Faith, Hope and Trick" (3.3), she lies to Buffy and Giles, saying that her Watcher is on  the annual Retreat in England. Both Joyce and the Scoobies - and even Buffy's potential boyfriend, Scott - are drawn to Faith,  who is flirtatious, friendly and "has rather a lot of zest", according to Giles. Everyone thinks Buffy is jealous of Faith, but they also note the new Slayer's wildness and lack of discipline. But she is also filled with self-loathing for allowing her Watcher to be killed, and it's not until she begins to patrol with Buffy that she is able to destroy the culprit.

Faith doesn't trust men, insisting they're all animals out for the chase. In fact, she doesn't really trust anyone, but is willing to risk her life to take down Wolf-Oz in 3.4, "Beauty and the Beasts", and when Scott Hope bails on Buffy before Homecoming, it is Faith who takes revenge, humiliating him in front of his date.

In "Revelations" (3.7), Buffy and Faith have become a team, but when Faith's new Watcher arrives, giving her the attention she's always craved, she's furious to learn that Angel has returned from Hell. But the new Watcher is not all she seems, and Angel turns out to be the good guy of the piece. But Buffy keeps reaching out to her, and is invited to chez Summers for the Christmas celebrations, ending up staying in with Joyce while Buffy deals with Angel. She even connects with Xander for a time, and they sleep together in 3.13, "The Zeppo" but then things start to go wrong when she kills the Deputy Mayor ("Bad Girls", 3.14) - a distinct violation of the Slayer rules. Buffy is devastated, but Faith becomes more withdrawn, insisting that she doesn't care.. Angel tries to talk to her, telling her he knows what she's going through, but then Wesley and the Watchers' Council capture her. She escapes, whatever trust she still had in others now shattered, and she turns to the dark side, becoming the Mayor's personal Slayer.

When Angel pretended to have been turned back into Angelus, Faith was eager to team up with him and torture Buffy, and her jealousy of Buffy became painfully obvious. Later, Faith shot Angel with a poison arrow, for which the cure turned out to be the blood of a Slayer. Buffy intended to feed Faith to Angel, but ended up putting her in a coma instead. Faith remained in a coma until the end of the fourth series — when she woke up and went seeking revenge against Buffy. Her method of choice was to switch bodies with Buffy via a gift left to her by the deceased Mayor. The experience was difficult for Faith, as she seemed to realize how unhappy she was with herself. She headed to Los Angeles and was hired by Wolfram and Heart to kill Angel, but instead she tried to get Angel to kill her. He wouldn't, and took her under his care until she turned herself in to the Police Department (she was wanted for crimes presumably including assault and murder).

Faith is played by Eliza Dushku.