This manga is so fucking awesome. That was some crazy ass battle. Some people assumed the ronin spirit thing was some sort of guardian, but I doubt it. It was probably more so utilizing Hana because it attracted a lot of other bad spirits, and it was eating all the ones that came. It was pretty much using Hana as a bait for food, but we're not sure if it would have eventually attacked Hana or not.
Furthermore, so it seems like you can pray and have some powerful spirits listen, but it seems like there's a debt to pay. We'll see if the story actually builds off of what happened here and has repercussions down the road.