i'm kind of confused? do you mean that billy would be undead in some way, like a ghost or a zombie, or someone who's stolen the real billy's identity, or that billy wasn't really there and bob was hallucinating somehow? or is the goal precisely to make it totally ambiguous? because in all those cases you'd write billy differently, provided that you want to hint at the truth along the way, which you probably should do, as otherwise the twist wouldn't really tie into anything or make sense.
a zombie could feel cold to the touch and look very pale, while an identity thief wouldn't know things that the real billy would and would get details wrong in questions. in terms of hallucination, bob could only see billy when no one else is around or other characters would act like billy wasn't there. it would also be good to give bob some reason to be hallucinating as it really doesn't happen unless something is physically or mentally wrong. for the totally ambiguous one, you'd mix different symptoms together in a way where each scenario would be plausible.
outside those, the character is pretty much free to be written in any way that a character can. being shy or boisterous doesn't really make you less or more likely to be dead.
