I don't know what Isshin's bankai will be. As for Zero Squad, it is the Royal Guard to the King of the Soul Society. It is made up of former captains that said that they wanted to retire and are never heard from again.
posted over a year ago
then it makes sense that Isshin might belong to zero squad. i've never considered it seriously so far
In some circles, it is actually believed that Isshin did not agree with the way that Urahara was treated and left the Soul Society because of this reason. Either theory could be considered valid.
There is a big chance that Isshin's Bankai shares some attributes with Ichigo's.
As for whether or not Isshin was a former member of the Zero Squad, I have to wonder how a member of the Zero Squad ended up in the world of the living when he his job would have been guarding the Royal family. It would be even harder for them to go then a regular captain.
