I think we still have some unfinished business with Ichiro. When Ronin killed Shiroyama, Roderico and Shatao, Ronin absorbed black smoke (the same black smoke that Sakiko uses when she tries to brainwash us) from their bodies. This smoke is probably the reason why Sakiko failed to subdue us (Sakiko even recognize Roderico inside us), perhaps smoke is reason why we were able to march for a month without stopping. I suspect that our true mission is not to kill the six spirits, but to collect them all in our body. If Ichiro ever returns (and he most likely will), it most likely will be done by hands of Mountain Demon.
However, all this is hust my hypotheses, and most likely I missed something, but sometimes it’s feels nice to just throw some text into discussion.