It's hard to say where Billes came into being, because he doesn't ever think about it. Whether he was genetically engineered, whether he's just an illusion, whether he was transformed into what he is now by some happenstance, all that is irrelevant. What remains is what he is. He is a toy, or sibling, or pet (or any combination of these), he is biological, and he belongs to everyone. He's not quite used to that last part yet.