In a fighting game, characters have their own set playstyles. You can't just play a character any way you want. (Well, I mean, you can, but you're not using them effectively.)
Take Dhalsim. He's the token range character in Street Fighter. His playstyle is pretty much poking you from afar because of his damn good range and keeping you away with projectile. His primary weakness is fighting up close, so he has to throw them away otherwise he's dead. He's not the kind of character you'd want to rush down with. Similarly you have Zangief, who is scary as hell up close but you really don't want to playing the poking game with him. Characters have set playstyles, even if they're not obvious at first. M.Bison is a heavy spacing character that relies on footsies and fundamental knowledge to rack up as much damage as he could despite his low damage output, Balrog is the defensive character that waits for an opening to punish you and punish you hard, and then you have Ryu who can adopt many playstyles because of his versatility.
Personally, that's how I determine uniqueness. If they can bring a playstyle no one else could, not about how their moves look or how different they are. Jigglypuff does, Mewtwo in Melee doesn't. Again, he can be fixed, but I'm just speaking in present terms. Right now he's barebones.
I'm pretty sure that's what I said. Yes.
I disagree. Ike is a veteran who has a unique moveset and thus is an established Smash character, so there is no reason to cut him. On the other hand I don't see any reason to add a character from a very popular franchise who hasn't appeared in a while especially when the series started going really strong AFTER said appearance.
There's really no character that you can compare K.Rool with. He's SUCH a unique and weird case to try and argue using comparisons.