It's a traditional horror movie style last second cheeky scare in much the same manner of 'Carrie', casting 'Leap Day William' as still a benevolent figure but with a slightly sinister side, much like the Halloween Jack-o-Lantern. We're told he lives at the bottom of the Mariana trench so his pike teeth are adapted for the fish he eats, possibly why he encourages Tracey to buy free seafood for everyone.