re-
Prefix (re-):means "back" or "again"
EX: react (prefix + root):"do back"
1. to act in response to something
2. to act in opposition to something
un- 1
pref. 1. Not: unhappy
2. Opposite of; contrary to: unrest
un- 2 prefix forming verbs
1. denoting reversal of an action or state: uncover, untie
2. denoting removal from, release, or deprivation: unharness
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So basically, Reverse/Rebirth just means Riku was "born again," possibly referring to Repliku, or possibly referring to his acceptance of darkness. Regardless, Repliku is probably completely, entirely, wholely unrelated to unbirths.
Now, unbirths, on the other hand...seeing the above definition, one would ONLY have to assume that "unbirths" are "the opposite of birth" or most likely....DEATHS! When death happens, the soul leaves the body, as said in one of the Secret Ansem Reports. So, unbirths may be the product of death. Just a quick, basic, highly flammable theory.
As for the whole paradox thing...that's what I've been wondering for a while. Heartless are "those without hearts" and yet...they have one...or ARE one....er...yeah. So that means unbirths "are" a birth? I guess? Maybe? Not really? Watevs. :blink:
So...critisim? flames? cupcakes? death wishes?