Eh I'll toss another theory in i only know anime stuff but I'll spoiler wrap it just in case.lol (I know i should read the manga, but i don't want to re-live claymore so waitin till the anime concludes or at least gets a bit furthur before i get into the manga)
Spoiler for just in case:
Isshin=ex-captain/royal guard/King (he told the grand fisher arancar that those of captain level or higher can control the size of their sword so he's least a cap, but i say he's either really old or of higher status or the captain of div 1 is in on him being down there since none of the other shinigami have a clue) himself went to the real world (obviously in secret). Either Ichigo's mom was a royal descendant, or the reincarnation of someone important and Isshin was sent down on a mission to protect her or Isshin himself was someone of importance and left b/c he fell in love with her. He lost his powers, or Uruhara helped him to hide there with a gigia till recently.
Uruhara & Yoruichi- Well being that they are the mad scientist and top covert ops leader, who else would be better to help Isshin, perhaps they knew each other before they entered SS, when they were "younger" and when Isshin left they helped and went with him. Altho Uruhara makin the cube (not gonna try to spell the name out.lol) prolly was a good reason for him to leave.
Ryuken-seein that he knows Isshin, and obviously knew Uruhara let his son escape, they are old acquaintances, and helped each other in some way. The ichigo's mom being his sister idea is lent credit if you think of what he said to Isshin, when he called him Kurosaki and Isshin commented that it was the first time he said that, so possibly some resentment towards Isshin over letting her die to the hollow (from Ryuken's POV he could blame it on Isshin) and by sayin the last name (which Isshin prolly took the last name of his wife) when talking to him showed an acceptance of sorts. He doesn't want his son involved in the shinigami prolly due to the genocide, and if his sis was ichigo's mom, also due to Isshin.
If 1/2 that turns out true then that would explain Ichigo's freaking reitsu being unlimited... part shinigami, part quincy, plus the hollow from him not getting his own shinigami powers fast enough..hell might as well dub Ichigo a "god"