A little image I've had in my head.
I've often wondered why Billy and Mandy have manadged to do what they've done. I've even concidered that maybe they're not purely human.
Here we see that they've both finnally reached puberty and wings have sprouted out of their backs. With this Billy's rather upset with grim for keeping his mouth shut, since he'd of been able to sence who they really were from the start. The tital seemed to fit.
I thought it would make sense to do Billy and Mandy each with a different kind of wings. They're so different already, and yet they'll always need eachother.