[christinemarie]

What if Isshi Kurosaki is actually the King's son and former no.1 captain (Yamamoto actually in a higher position back then but force to return to 1st since Isshi run away. In order to protect his son he ordered Yamamoto not to tell about it, he doesn't know yet Ichigo is the grandchild of the King) before the old man take charge but runs away since he hates responsibility and went into the Human World after what happened to Urahara and others and stayed for many years until he met Ichigo's mother and everything happened as it is. That explains his power.

Other theory Isshi is actually the missing King of the Soul Society. That makes Ichigo prince of Soul Society and his sisters princesses.

Isshin Kurosaki is Urahara Kisuke's master who teached him lot of techniques. He's the only Captain that can use 3rd form of his Zanpaktou more powerful than Bankai.

(Although Aizen also knows how to use the 3rd form as well, the real villain in the series is Aizen's teacher. He get more power by using the hollow's abilities. And want to get King's Key in the process to further making him powerful.)