>>27853644Greninja gains its trainer's features. It's not literally hair, just the shape of the fins changing.
In biology a protean animal can similarly alter its flesh. I have no idea why they decided to give it to a frog, but it's consistent with the term. Besides minor trainer-based adjustments, it just gains the giant shuriken, which is basically an objective improvement. You don't need to heavily alter its design.
There is no inherent value in giving it a mega form vs giving it some other powered up form.
