I think Isshin was a Shiba, and he's the one who threw the family into disgrace because he abandoned Soul Society in order to marry a Quincy, a sworn enemy. They decided to hide in the human world and retire, possibly in order to hide Masaki from Juhabach. Ryuken was probably in love with her and he blames Isshin for her death, that's why he hates him. Ichigo and Kaien look so similar because they're blood-related, and the reason why Kukaku and Ganju didn't comment on the obvious resemblance is because they knew the reason.
I'm calling it.