I dunno, it kind of looks like the lovechild of Altair from Assassin's Creed and a steampunk G.L.A.D.O.S. from Portal. Which, I think we can all agree, makes for one hell of a sexy enemy! *wolf whistle* Hey Big Sis, come get some of this! Haha, I know that sounded wrong but I'm not taking it back.
First Image of BioShock 2's Big Sister [kotaku]
Thanks to Julian and junkyard dog, who find both find the Big Sisters even more attractive than I do, which makes them both sick in the head.