Luke, realizing that just clawing at your surface is useless, stops to look at his friend. Eying the small purple fox balloon that floats at the ceiling, and remembering just how it got there, he breathes deep and tries his best to hold his breath, keeping you from forcing the same fate on him. Unfortunately, he doesn't seem to realize that the air he's holding in came from you anyways.
Having done all that you really needed to (for now, anyways), you shoot him out with a massive gust of air, quickly tying off your knot and wrapping your string around Luke's wrist to make sure that you get a good view of the action. He tries desperately to get it off of him, but has no better luck than he did escaping you by clawing at your skin. Breathing a sigh of dismay, he starts towards the door of the bathroom... and then stops. He looks down, noticing the present that you'd left for him: the air he breathed in while he was in you is still in his chest, keeping it puffed out. The look on his face when he sees it is priceless, and if you had a mouth you'd probably be laughing right now, but even that doesn't match what he does when he lifts his shirt and finds that his chest is not only puffed out but that it's also mostly smoothed over, having turned a shiny pink color. He starts breathing faster, trying to get the air out of him and get his chest back to normal, but it does nothing. Of course, that's not the end of it.
Just as you'd hoped, he clutches at his chest, squeezing at it. To his surprise, it turns back to normal... as the air rushes away from it in a cartoonish way, shooting downwards to puff out his stomach in a similar fashion, then moving out of his stomach and into his hips, the bulge then splitting and traveling down his legs until finally causing his shoes to burst with a pair of Bug Bunny-esque pink rabbit paws and coming to rest at that. More panicked than he's looked all this time, Luke's eyes start darting across the room for any possible solution to this. Meanwhile, you hover gently above him, delighted in your work and thinking about what you should do next. Maybe a nice, rubbery tail to go with those feet? Or a pair of tall ears? Of course, there's nothing wrong with a cute little pink bunny nose either. So many choices!