The problem with introducing Mokou is -how-. Unless it's a bodied-type simply coming to him (perhaps a business card was passed and she wants to move in?) As a yukkuri, she is an immortal pyromancer. I could set up a random encounter, yes, but then what to make of it? Rin can make fires, maybe Mokou could help with that. It hasn't come up, because Rin hasn't been under that much stress. Mokou could help her bring that out, I suppose. For what, though?
Then we return to the question of how they meet. Perhaps accidentally started a fire, they tried to dispose of her, but couldn't, so they bring her to him (ref: cop on the beach?). I dunno. Have to sleep on it. Then comes chronology. You have to understand: there's a lot on tap. Let me know if you -ever- want a peek down the rabbit hole, and I'll give you some ideas of what's coming.
FYI, the longer I take on chapters, the more I can flesh them out. My subchapter 7.3 was supposed to be about shopping.
I literally just had an idea fall into my lap WHILE they were shopping.