There is no such thing as death in the kh universe, everyone just comes back as a nobody, somebody or get trapped in the RoD. That's pretty much disneys fault.
If I had a hundred dollar bill for everytime i've heard this I would be Donald Trump.
The process of creating a heartless and nobody is not the same as dying. You don't die if you lose your heart.
When a heart is lost to the darkness it becomes a heartless. The body and soul left behind can sometimes become a nobody. When the heartless is defeated and then the nobody is defeated the two beings somehow become whole. (Though how a nobody and heartless become has not been detailed yet)
Death occurs when the soul leaves the body. Since the soul is like the fuel that runs the body, death pretty much is when the body runs out of gas.
Heartless and Nobody Creation = Loss of heart to darkness
Death = Loss of soul/Body runs out of life energy
Characters who have died in the series:
1. Jafar
2. Oogie Boogie
3. Barbossa (Though if Port Royal is in KH3 we know he'll be back)
4. Ursula (Twice. Seriously how did she come back in KH2?)
5. Clayton
6. Hydra
7. Mufasa
8. Scar
9. MCP and Sark (Do they count if they are digital beings?)
10. Lady Tremaine and the Step Sisters
11. Repliku (Maybe)
and I could go on with other examples but you get the idea. Its most disney characters/villains who have kicked the bucket. Almost all of the main characters have never met the requirements that would = death.
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As for who is most likely to die in KH3? Probably MX because it is the end of the "Seeker of Darkness Chronicles" and MX is the main villain of those chronicles. Terra maybe, if he pulls a sacrifice to stop MX in KH3. I could see Yen Sid getting killed in an Obi Wan Kenobi type of situation. And maybe Riku...just a feeling I got.