User blog comment:Godisme/Enemies in the Dark/@comment-1706259-20111116205013/@comment-117598-20111116213226

Captain Brooks said Isshin COULD be a substitute shinigami... didn't say he was. We don't know anything about his backstory, so anything is possible. Does it need to make sense to the readers? It would be nice, but as long as it makes sense to Kubo, he'll write it out how he wants to. It is his story, after all, and no one is obligated to buy it if they don't like his way of explaining things.