Maybe if Mitori (not Nitori) was there, it'd make a little more sense. Since Yuugi is the fake mid-boss of the fake Phantasm stage. But I agree, this is...a very strange pairing.
The oni used to reign over Youkai Mountain, meaning they were the kappas' and the tengus' superiors. At least Aya has lived when the oni were still around. I don't know about Nitori's age, though.
Also changed to explicit because of that fluid in Nitori's pussy.