Originally Posted by kael03
The only shinigami that seem to remember who Isshin is are Yourichi, Urahara, and Aizen.
Being that those are the only characters that have said anything (or made an inference) to Ichigo about Isshin, it makes me think Isshin was Zero squad.
It's been stated that Urahara has been to the Zero squad's land (copying the hot bath thing). I'm beginning to theorize that Yoruichi has always been a liaison to the Spirit King, and Kisuke was her top operative (or even he was possibly Zero squad).
This is one flashback series I really wish Kubo would write... there is just so much potential with Isshin.