The Death System

Death:
So your character was struck down, but worry not! That's not the end of your adventure! In fact, you have a few options as to what happens... Detailed below!

Heartless:
Yes, if your character is whole and they perish, you can then take up a Heartless. This would be for characters of a more evil alignment, though there's no true restriction in regards to this, as Lil' Shadow Sora was hardly holding ill intentions when he tracked down Kairi in Hollow Bastion...

If your character dies as a Heartless, you have the option of becoming whole again.

Nobody:
You can also pick to be a Nobody, this is effectively the "catch all" scenario. Your character isn't restricted to being one of the Dusks, however. You can start with human form and powers. And just like with the Heartless, if you are struck down you have the option of becoming whole again.

Lingering Will:
If your character possesses enough Willpower, on death or loss of their body they can take on a Lingering Will. This most often resembles their current peak of power, in some cases they can be even stronger than they were in life. Lingering Wills have the option of hunting their respective Heartless and Nobodies in order to become whole again.

Or otherwise defeating their bodysnatcher to claim what is rightfully theirs...

The Chain:
When all else fails... There are those that hold the power to manipulate the Chain of Memories. These beings are capable of severing ties, manipulating one's subconscious... Or making them intact once more. Whether Heartless, Nobody, Lingering Will, or even Whole, this is the one guaranteed and perhaps most reliable way to achieve what one deems to be their... 'Ideal' state.