Board Thread:Questions and Answers/@comment-11927664-20130730160308/@comment-44061-20150328065402

Isshin's last name is Shiba. He married Masaki whose last name was Kurosaki which is a Quincy family name seeing as she was a Quincy and he basically took her name when they married and went undercover for the past 20yrs. No one knew where he was. Prior to that he was a branch head of the Shiba Clan and the former 10th Division Captain before Hitsugaya. In Japanese nobility a branch head basically means he's the younger brother of the main family head and thus he is Kaien, Kukaku and Ganjus uncle (which they insinuated when they sent ichigo to the royal palace with the royal guard) and therefore Ichigo, Karin and Yuzu are their cousins.