User blog comment:Nick D Wolfwood/Questions questions questions!! Kurosakigate.../@comment-44061-20100624153219

The idea that Isshin was a captain is a strong one. Whether or not he was the captain of the 10th division is unknown. People put it out because Hitsugaya is the newest captain and with the 20 year de-powered period its generally thought it perfectly coincides. The issues are that no one recognizes the name Kurosaki nor does Aizen himself seem to really be familiar with Isshin besides acknowledgement of him as a Shinigami and Ichigo's father.

Which would lead one to believe that he predates Aizen's term in the Gotei 13. Who knows at the moment we will see eventually.

The other points are he is very powerful more so to the point that he can easily take on Aizen and accomplish feats beyond an average captain. He also was privy to knowledge about the Arrancar, Vizard and Aizen where most lesser know Gotei 13 members may have not been aware. Especially considering he had been powerless for 20 years. He also has considerably knowledge of SS and Shinigami practices.

What we do know is that his Zanpakuto is similar in name to Ichigo's and that they are both capable of similar attacks. But they are considerably separate. This isn't really a confusing issue. Zangetsu (slaying moon) and Engetsu (crescent moon). Getsuga Tensho (Moon fang heaven piercer) is a very simple attack that condenses reiatsu and releases it as a destructive slash of energy. The acknowledgement would be that families possess similar zanpakuto in abilities. As we haven't seen any other families in the series its hard to register. Though different they are possessed of common traits just like children possess traits of their parents and grandparents. We need not forget zanpakuto spirits are parts of their wielders souls. Their powers and personalities our reflective of their wielders even though they are independent. So it actually makes sense that Ichigo who is Isshin's son has something similar to his father other then genetics. I would think of it as similar to spiritual genetics if that term is usable. Similar but different.