Unfortunately, I don't think there's a Wiki! The closest thing is a TV Tropes page, but that doesn't talk a ton about Robert.

Robert is more something else entirely; he's not a former human in a transformation sense or a ghost sense. Early in the canon he lies about being a cursed doll brought to life by voodoo, but he's actually just a straight-up monster. In the world of The Last Halloween, monsters live in a shadow dimension that's sort of beside and connected to the human world; every time a human is born, a monster comes into being that basically embodies their deepest fears, and they can take any form at all. (This is the main character and her monster!) When their human dies, the monster dies as well, unless the monster is the one to kill their human, in which case they become basically immortal. But despite that connection to humans (or one particular human each), Robert himself isn't and has never been human in any way at all.

Thank you for discussing this! If you have any more questions I'm happy to answer, and whatever you two decide I definitely understand.