More from Kubo's Masked interview with permission from pony_rocks (a professional translator btw). I don't have the verbatim translation yet but she allowed me to post this info from the Byakuya FC at BA (post here):
Coincidentally, I've just been translating the Masked interview with Kubo for myself and it says that Soujun Kuchiki died in a battle. Apparently, he was rather delicate (the usual "his body was weak") and not really battle-oriented, but he still possessed some talent and after all he was supposed to become the next head of the Kuchiki clan, so he tried hard and made it to the rank of vice-captain, but then got killed. Poor thing. Kuchiki boys are obviously delicate.
Ah, Byaboi and his delicate flower petals of doom. Now I'm wondering what his daddy's zanpakutou was!