JAIF

Eh, I think if they are killed, they disappear completely - after all, their souls are destroyed, everything that connects them to existence is severed. If they were just going to be reincarnated, death wouldn't be such a dreaded event anyway. I do recall it stated somewhere, Rukia telling Ichigo he'd vanish completely if he died in Shinigami form, but can't remember where, exactly.

As for natural reincarnation, I'm inclined to believe it's a matter of choice. After all, Genryuusai has been in Soul Society for ages.

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