User blog comment:TheBody84/Ichigos father being the former Kenpachi/@comment-1454424-20091025103705

As Twocents said, there is ZERO chance of anyone alive being a former Kenpachi. I am a firm subscriber to the theory that Isshin is royal guards.

However, if he was Gotei 13 then he would be Captain of Squad 10. The position of squad 10 was vacant during the turn back the pendulum arc and we have no idea who the captain(s) were between the turn back the pendulum arc and Hitsugaya taking over.

Moreover, assuming that "Kurosaki" was not Isshin's last name when he was a captain, who is the one person who would recognise him. Matsumoto Rangiku and Matsumoto has never seen him. If Rukia didn't know the captain of the 10 squad very well, there is a chance she wouldn't make the the connection automatically given the altered context. It is surprisingly easy to "fool" someone just by altering a the context. I.e. if you are used to seeing someone in uniform, you might not recognise them when they are not in uniform and often they would have to explain themselves for you to get it, e.g. "I am Bob! You know, Bob the security guard you see everyday!"

Also, Rukia hasn't exactly spent a lot of time staring at Ichigo's father. She stayed with them a couple of days before returning to SS with Orihime and then coming back for one, maybe two nights before leaving for SS again and then following Ichigo to Heuco Mundo. So her not recognise him doesn't mean much. Ichigo's father has not been seen by more "senior" members of the Gotei 13 like Matsumoto, Yumichka or Ikkaku. One of them might recognise him, specially Matsumoto if he is indeed the former captain of squad 10. Heck she might even have served under him!

But I mostly think he's royal guards.