im just spit-balling here cuz i've only heard of this a two months ago but didn't believe it until two days ago. i have read a few other sites yesterday and peiced them together to come up with my conclusion. (i don't care if you think this is copy-write or whatever) i know that the big daddy toy is the replica of the first proto-type big daddy ever made (you get to play as him in the second game) and that the girl has telekinesis powers from when she was in Rapture. the two must have met each other and something happened between the two, like the proto-type Mr. Bubbles saving her but then they were separated and she can barely remember him. the ten years pass and her powers start to emerge as she begins to remember Rapture the way she knew it and she wants to reshape and repair it into the best city in the world, just like Andrew Ryan wanted to do (i think). she returns to Rapture and becomes the "Big Sister" that takes, possibly, the duaghters of the origional little sisters and use them to replace the missing ones from ten years ago. The Big sister rules Rapture, but not for glory or greed, yet she is savage and merciless to any that dont follow her rules. As the first big daddy that is not just all brawns like the other models, you start to realise where this city is going to end and you want to stop the misguided big sister before she brings the distruction of the entire world. but i think that the blue butterfly crawling out of the barnicle means hope from unexpected places.