Warning: Wikipedia contains spoilers.
Faith is the third Slayer to appear on Buffy. She came to Sunnydale in season three having been granted her powers due to the death of the previous Slayer, Kendra.
The relationship between Faith and Buffy is a complex one. Originally they fought side by side as allies, but there were always hints of tension and jealousy between them. The relationship deteriorated after Faith accidentally killed a human, and eventually, seemingly with no friends left, Faith joined the forces of Mayor Wilkins, the Big Bad of Season Three.
After mortally wounding Angel with a poison, the only antidote to which was drinking the blood of a Slayer, Buffy hunted Faith down in an attempt to use her as the cure. The resulting fight left Faith in a coma.
She awoke in the middle of Season Four, and used a magical device to swap bodies with Buffy. It was during this period of acting as Buffy, the "good" slayer, that Faith's attitude started to noticeably change for the better.
Later, with Angel's help, Faith came around to a better understanding of who she was, and became more at peace with her place in the world. Faith helped save Angel in Season Four of Angel, and played a huge part in saving the world in Season Seven of Buffy.