My guess (I haven't actually read ahead further than 34, that should be done by Sunday maybe) is that American Evil Repelling Totem-kun used up all its power (and then broke) to make sure Mogami was there and would save Goro this time. Which would mean Mogami isn't necessarily a friend of Kogane's family, though she sure does seem non-human - but she obviously wants to protect Goro, maybe just as a subordinate she likes, or just as a decent person. I do like the oni theory! And of course it's possible that I'm wrong on American Evil Repelling Totem-kun and it was just broken by Mogami's killing aura, even though it was aimed at Stalker-kun.