Ditto. It would have very weak attacks but when it hits an opponent it transforms into it, like Kirby except Ditto is completely identical to the character, except in color to identify the two, and could transform back by taunting.
Or better yet, since there are three different taunts, each taunt would transform Ditto into a different character.
Ditto's Final Smash would have Ditto transform into a legendary Pokemon, similar to Bowser becoming Giga Bowser. Maybe Mewtwo if it isn't playable, or some legendary fro XY.
