User blog comment:Godisme/Everything But The Rain/@comment-1837917-20130228054630/@comment-4794735-20130301052248

My theory has always been that Isshin is the former 10th Division Captain that proceeded Toshiro. My reasoning:

-For one, there's the fact that Shinji didn't recognize his reiatsu, which would make sense if Isshin filled the vacant 10th Division position after the Visoreds left.

-Secondly, and this is where it gets tricky, we know Isshin lost his powers 20 years before the main story (approximately 22 years before where we currently are). Toshiro is the youngest Captain in Gotei 13 history and we know from his back story that he became Shinigami while Hinamori was in the Shin'ō Academy. Hinamori attended Shin'ō Academy with Renji. During Renji's time in Shin'ō Academy was when Rukia was adopted into the Kuchiki family, which took place 40 years before the start of the series. The curriculum at Shin'ō Academy is taught over 6 years, meaning Rukia's adoption took place between 40 and 46 years before the series began. We must also take into account that Aizen was 5th Division Captain during Momo and Renji's time at Shin'ō Academy, and that Gin was considerably older (appearance-wise) Through all this, we can determine the following: --1. Toshiro joined the Shin'ō Academy roughly 40-46 years ago, approximately 50 years after the Visoreds incident. --2. Because he didn't join Shin'ō Academy till then, there's about a 20 year gap between when Toshiro joined and when Isshin lost his powers. --3. BECAUSE OF THIS, there's roughly an 80 years gap after the Visords left where Isshin could've filled the vacant 10th Division position, which would explain Shinji not recognizing his reiatsu. --4. And, even with all this, there's still plenty of room for Toshiro to fill in the once-again vacant 10th Division Captain's position less than 20 years ago, which would be rough 20 years after his graduating Shin'ō Academy (which we can assume 20 years from new graduate to Captain to be somewhat impressive).

So for all of you tl;dr people, here's the summary of my theory: Isshin became 10th Division Captain at some point after Urahara and the Visoreds left, then lost his powers 20 years ago (possibly via Saigo no Getsuga Tensho), allowing Toshiro to take over his position.

Feel free to comment.