Kyonyuu Fantasy/Headscratchers

Yes, this is a fantasy nukige visual novel H-game with a 2 episode anime adaptation, but it still has a surprisingly interesting story, which begs the questions...

WARNING - Many spoilers are not marked here!

The Racial Feud between Humans and Demons

 * Both sides are said to have ample reasons to view each other in positive AND negative lights, but there are so many parts of it that seem to be contradicting, such as King Hagel I marrying a Nymph and keeping her identity as a demon (as well as their daughter's identity as a half-nymph) a secret from everyone rather than just rescinding and removing the "Demon Extermination Order" like Lute does when HE eventually becomes king so he could openly marry Shamsiel?


 * How is it that if demons are so powerful (which they demonstrate many times throughout the story), they've nearly been wiped out by humans?
 * One may think that perhaps they aren't as "extinct" as humanity thought. Plenty of demons encountered in the story had been living away from human civilizations.
 * There's also the idea that some demons took to hiding in plain sight among humans (such as Luceria's mother).

Is King Hagel I part-demon?

 * The story reveals that his daughter, Princess Luceria is half-human, half-nymph (a type of demon). It goes on to say that thanks to this, Luceria's body has lethal magical properties similar to that of a succubus, so any normal man who gets intimate with her would either lose their mind or die. Lute negates this (after struggling initially), thanks to being half-incubus which is the highest class of demon in their world. However, how then was King Hagel I able to survive being with his 2nd wife who was actually a genuine Nymph? It is said that Bobon is a normal human (though his demeanor suggests he might be part-man part-stupid-cow), and he was King Hagel I's firstborn son from his first marriage to a royal family member from the Lingobard kingdom who (as far as we know) was a normal human too. Again, what was it about him (a supposedly normal human) that made him able to survive his relations with a full-blown Nymph for years?