I agree that SK is the most obvious choice but keep in mind all we have is Cary's theory and there is no guarantee that he is 100% right. Entirely possibly that this is a personality of David as Cary's theory isn't actually supported by anything really scientific in the show as I see no mechanism for him to be able to distinguish between a parasitic entity and a split personality that was so powerful that it appears to be a separate entity within David.
Hell they could even go Primal Fear on us and make David be the split personality and this other more sinister SK like entity being the real David. But again SK seems the most likely but this show is show fucking out there, I think anything is possible.