Faith is the dark image of Buffy. She didn’t grow up with the love Buffy experienced, she has different issues with liking and leaving guys, and she was in a rougher place when she was chosen. It doesn’t mean that she’s bad. She slipped like everyone does (and can), especially in the Buffyverse, and she paid for it. She lost her only friend and exiled herself from the outside world. Then she gets busted out by Wes, whom she hurt (badly) and goes to fight for the good guys back in Sunnydale
Faith is the Angelus to Buffy’s Angel character. Like Angel/Angelus, one character isn’t singularly good and one is evil; its more than one happily leans one way while the other struggles down the path of righteousness. As its been pointed out, Buffy isn’t always good. She has slipped the line a couple of times. No one is perfect all the time. And no one is bad all the time.
I love the character of Faith; she’s everything that Buffy maybe wants to be but could never be. She’s sexy, flirty, sometimes careless, tough, and she’s a brunette for Pete’s sake, which clearly shows that blonds don’t always have the must fun.
All that said, she grows out of being a vehicle for the third season plot about the power of being a slayer and becomes a full character in her own right. Faith finds a father-figure of her own in Mayor Wilkens, much like Giles is Buffy’s pseudo-father; their relationship, when it is looked at without the “evil” involved, is pure and sort of sweet. He cares for her completely, as a daughter, and even looks after her in death. For Faith, she finally finds a man she can depend on; no doubt she had daddy-issues of her own before coming to the Hellmouth, leading to her feelings about men. Wilkens doesn’t ask much from her, just to kill a few people, poison Angel (partly revenge for Angel not going with Faith earlier), things that Faith is willing to do anyway; he helps focus her energy, her powerful slayer gifts, for the evil side. He has been alive for 100 years, lost his wife when she died naturally, and might just be lonely even if he is going to be a pure demon. This is something that has been standard from season 1, villains who are just as human as the heroes. The Mayor may be evil but he is also loving and caring, just like everyone else.