Faith led a fairly traumatic and abusive life, and not much about it is known or discussed throughout her appearances on the show. After the death of the Vampire Slayer Kendra, Faith is summoned. After losing her Watcher, she comes to Sunnydale and attempts to fit in with Buffy Summers and her group of friends... but it becomes clear over time that she doesn't really mesh with them.
When out patrolling with Buffy one night, Faith accidentally kills a human whom she mistakes for a vampire, and her reluctance to confess to the crime further alienates her from the group of people whom she once wanted to be friends with. In a move of desperation, Faith allies herself with The Mayor of Sunnydale, and causes chaos all over the place in her determination to find somewhere where she belongs. Upon an attempted murder of Buffy's lover, Angel, Faith is brought to "justice" when the Slayers battle it out, and Faith loses. Blood loss knocks her into a comatose state that lasts several months.
Upon awakened, Faith is shocked to discover that everything she built up for herself has been torn down by Buffy Summers, whom she claims to despise. She learns that the Mayor, whom she came to regard as a father figure, has been killed - but he left her a present. A Body-Switching Device, which Faith uses on Buffy. While Buffy is forced to ward off the Watcher's Council in Faith's absense, Faith learns for the first time what it's like to have loving friends and family, and a lover who really cares about you, and it changes her perspective. Rather than immediately fleeing the state, she chooses instead to stay behind and help captives in the church, which leads Buffy to her. They switch back, and Faith flees Sunnydale once again.