lol... that makes the big daddies sound like mobsters, almost o.o
but yeah, apparently you go rogue and start fighting against one of the little sisters from the first game who grew up, put herself in big daddy armor, and now is going by the name of "big sister"... i think that's her backstory at least... either that or she was just an alternate form of the big daddies... there's a site for it
http://www.somethinginthesea.com/ <---there