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My theory on Isshin is that he was the 11th division captain.
We do not know when Zaraki became the captain, but we know that he needed to defeat a captain to become one, in order to avoid the captain's exam. My guess is that Isshin was the captain that Zaraki defeated. Perhaps, the duel was staged. Isshin wanted out of SS.
We all know he cannot be Kaien because Kaien is Aaroniero Aruruerie.
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