Have you compared the animations from sora and riku?
all the sequences i've seen seem pretty Generic. the chr modles don't even move while a sommon is doing it's thing. sora can even use micky, so i dought it would cause any problems. it's most likely one summon animation for all of them. the hard part would be getting the cards into riku's deck.
I'm sorry for my error in terminology. The Ansem Reports do say soul, not mind. my confusion came from nobodies having no emotion, only memories. it made mind seem to make more seance.
Still, heartless do operate on instinct. While there opposite, the nobodies operate on memories. The only thing that hasn't been done yet is something operating purely on emotion. something borne directly from the heart rather than from loosing your heart.
This is just my opinion, but you seem to be over thinking the empty space a bit.
the origin of this symbol is more obvious if you brake it down a little.
The unbirth emblem is composed of four horns (the musical instrument, not like a goat horn). the four horns are forming two X shapes, which are positioned to make the three simple shapes used in a heart symbol.
In KH2, X was supposed to represent ware light and dark cross, or reach a balance. we can draw from this that the unbirth have something to do with the balance or connection between light and dark.
now, this doesn't tell us much about what they are, but the heartless and nobodys can. the nobodys are the flip side of heartless, a mind without heart or body, even there symbol is only a slight varient of the heartless's, just fliped upside down. that accounts for two thirds of a human, body and mind. but the heart or soul(two words for the same thing) goes unaccounted for. so the unbirth is likely a soul without body or mind. no thought, no instinct, just raw unbridled emotion.
