Mostly different. In his first appearence, while their playstyle was a bit similar, the way they actually played felt completely different to me, regardless of the similar moves. If anything, they play the most differently from the other 4 characters because of the different abilities they possess.(for example, Shadow has no Bounce Attack, completely changing his strategies and how he goes through everything)
In multiplayer, yes, they're pure out clones, but 1Player is what seperates every character.(not counting the timing, etc. on Chaos Control and other Multiplayer abilities)