Both Ichigo and Kaien are form Shiba clan but their relation is not explained yet. Kukaku may know Isshin, after all Isshin was Captain and probably head of Shiba clan. Why Shiba clan demoted from the rank of Nobel is also not explained yet. Ukitake, Byakuya, Rukia, Kukaku, Ganju, everyone was surprised by the striking similarities between Ichigo and Kaien. Yes, there are just too many questions unanswered. If only Kubo give us flashbacks of Yamamoto vs Yhwach and explain the origin of Noble Clans. Hope that Kubo won't leave these unanswered questions like Kishimoto did with Naruto.
it is a plot hole for now.....everyone seems to have forgotten this connection for now at least
Kaien an ichigo are from the same family so don't that mean they are related?...I'm pretty sure kukaku knows isshin sense he used to be a captain an went by isshin shiba.....an when she sends ichigo off to the royal palace she says uncle might not like this an I'm pretty sure her "unlce" is ichigo dad...cuz I see no point in bringing up her " uncle" if she did not know ichigo an her were related...cuz I'm pretty sure ichigo dad didn't want ichigo to find out the truth about his mom seeing as he stopped aizen from talking about it or he knew if ichigo got to strong he would not be able to return to the real world like kyoraku told ichigo friends....
