[Siclife6]

I believe isshin is related to the shiba clan maybe brother or cousin also believe that Isshin might have been one of the kings royal guard however his time as a royal guard ended with the death Kaien Shiba. What I mean is when he found out about Kaien's death he may have abandoned his post as royal guard their for disgracing the Shiba clan maybe he was capture by 13 court guards and put on trial by central 46 his punishment was to be stricken of his powers and banish to the real world for his rest of his life. Note this is the same punishment that was going to be used on Urahara if we recall. As far as his relationship with Ryuken Ishida and just talking out of my ass maybe ryuken is related to Masaki Kurosaki maybe sister this would explain Ichigo's Kurosaki strength that would make him pure blood shinigami and a quincy that would explain why Isshin and Ryuken were questioning each other's parenting methods. Well thats my theory if u have something to contribute to make my theory even better help me out .