Just to put on a more clear example.
If Spider Man appears on a x-men issue, he is not a "guest character", he is not a special appearance, he belongs there.
Now put batman on an x-men issue, that doesn't work, he doesn't belong there (and they tried to crossover marvel x dc once, it was a mess).
Now, you might say: "but naotora is owned by KT", true, so let's say this way...
Does Elsa from Frozen belongs into the x-men universe? No! They're both owned by disney, but they're not the same IP, not the same universe.
That is the difference, and it is pretty clear, Rachel and Momiji = Not guests. Naotora = Guest. Is just a fact.