REGISTER TO REMOVE ADS
It's nothing Solid, but I do believe that Memories can contain enough of a Sense of Self to create a Being. If a person also was to lose all his Memories, it would be like he never even existed - thus, Unbirths.
You're basing that on Maleficent's resurrection, aren't you?
1) The person on itself wouldn't have "Existed".
2) Namine taking apart Sora's Memories and making for everyone else who shared those Memories to forget them too. It's a link between people, not a copy in each person's Heart