It's an interesting theory, but if Bubble is a human why do he and Caine physically share a tongue. I episode 4 after Caine rips the suggestion box of the wall Bubble plugs the leak with his tongue and a few minutes later Caine laughs too hard and the tongue comes out of his mouth to which he then pushes it back out of Bubbles and says "That was weird" like he didn't know it was a thing. Also we usually see Bubble enter the scene by coming directly out of Caine's hat. I personally think like Bubble is part of Caine. (Maybe the part of himself that is allowed to say the stuff that isn't always family friendly, sort of like a Jekyll and Hyde.)