What, do you think this is Enterprise or something? Not that T'Pau and Hoshi in various states of undress is a bad thing. And if Shannon, Sun and Kate were to have a half-naked catfight, I'm not gonna complain either.
Sun's Korean Yakuza/Triad-equivalent father was subjected to cosmic rays during the crash, mutating him into a large, horrible carnivourous creature.
If we go the Keyser Soze route, it's probably Hurley. If we go for someone more obvious, then it's Kate.