Anyone float the theory that the MC is actually Katherines son?
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I mean, no one knows who his mother is... and Katherine and her daughter are the only two people rare enough to birth an all powerful demon spawn. It would make sense that the MC is that son in this fantasy world. Maybe grandpa erased her memory of birthing him when he concealed her... And the gramps clearly sent the MC to get to know Katherine and her kids because there was no real threat.
That revelation would be disappointingly obvious. Long lost child is frankly an overused cliche. And it's risky with patreon's policies. They are too anti-incest to allow a threesome where only the girls are related to each other.
MC's mother being the same kind of demon as Katherine is practically guaranteed though, but even so they might not be related in spite of the kind being rare, because logically it shouldn't be.
1. They are highly priced by higher demons, so there should be plenty of protection.
2. The seductive aspect of their power should give them more protection.
3. Girls always inheriting their mother's power should make them relatively numerous.
It's something of a plot-hole. I can think of plausible explanations, but until the game offers one that's just speculation.