User blog comment:TheBody84/Ichigos father being the former Kenpachi/@comment-1403595-20091025074819

10th squad captain is most likely. We know so little about the 0 squad that it's impossible to say whether leaving is even possible considering it's in another dimension. We have a few clues to Isshin though. As has been mentioned, Urahara said something along the lines of "it'd be a pity if you blamed me for losing your powers in the first place" or something, so whatever reason he lost his shinigami powers for, it had something to do with Urahara. Now 20 years ago, Urahara was living in exile in the real world. The other thing we have to consider is that Isshin knew about both arrancars and the vizard. We know that none of the current captains knew about the vizard, because of their surprise at seeing them arive in fake Karakura Town. So Isshin found out about them after he left his former job. Therefore, the probably story is that he was in the real world, investigating something, who knows what, and he got mixed up in the vizard and Urahara who somehow caused him to lose his powers. Therefore he hid out in the real world, which is possible because most captains are capable of hiding their reaitsu (or even more likely he had little to none on account of losing his powers, explaining why he, unlike his children, could not see plus spirits at the start of the series). Now, he has recovered his shinigami powers, not sure how.

An alternate explanation has to do with his aquaintance with Ryuuken, and being involved with the remaining Quincy, although that explains less and it's probably that he knew Ryuuken due to some other means.