Oh, I think Faith was definitely a second try at a foil. She's almost the anti-Kendra, but both are the anti-Buffy in different ways. Buffy is meant to be a different kind of Slayer because she has friends and other connections who help her in her fight against evil, and because she bends the rules. Kendra works alone, and follows all the rules to the point of inflexibility. Faith works alone, and breaks every rule she can. The difference is we find out why Faith works alone — she's a damaged person and only trusts herself — whereas why never find out why Kendra is the way she is. At the same time, Faith offers Buffy a very real alternative way to be a Slayer, and Buffy is tempted at times to follow her down that path. Kendra never offers Buffy a serious alternative. We're meant to approve of Buffy's choices, but Kendra seems to exist solely to represent such a choice, whereas Faith is a human being as well. And this was a failure on the part of the writers. We could have had Kendra's backstory, we could have found out why her community treated her Slayer calling so seriously, and why they felt she needed to be isolated and trained from childhood to be a Slayer, but we're never given the opportunity to do so.

Does that make any kind of sense? (Also, sorry for taking so long to reply to your comment.)

Sorry to take so long to reply