@Hachiro Terra
Alright, first thing's first - your abilities. That's the core of the problem.
If we're going to make the character more original-original, we'll need to remove all aspects that originate from Kingdom Hearts, such as Keyblades, Organisation XIII, the mook transformations and stuff.
Replace them with similiar abilities that aren't too overpowered.
Here's my suggestions:
Change the name of your split-forms. Well, since I'm planning to play a self-replicating character in future I probably shouldn't tell you to completely ditch it. Maybe add some weaknesses, such as each copy acting too independently, or each copy sharing a lifelink, or even a time limit of sorts!
But yes, if you want varied abilities on each replica, reduce the summoning power of the Nightmare.
Well, those are more 'familiar summoning' than 'self duplication' really.
Instead of a keyblade, why not a legendary sword? Or even a weapon that resembles a key but isn't a keyblade? A shield with a lock in the middle for a better motif-fit? Maybe connecting rhe lock-shield and key-weapon will unlock some form of greater power with side effects or a time limit, explaining why he doesn't do that all the time?
Additionally, take advice on which abilities are broken and fix them.
Next - your history.
Since it has to do with Organization XIII, you'll need to change it. Need suggestions?