I suspect the old demon king know it and view him as a threat but afraid that if he kill him it will just back-fire and kill him or any thing will just be a self fulfilling prophesy so he made him so attached to him he would be reluctant to kill him.
Man this seems fun but...does he hasto be great at everything? I hope next chapter shows he isn't great with the defense aura and gets wrecked by the dad's attack.
As it turned out he did not have the defense buff and tragically died to the dads attack.
Without their foretold hero the humans were powerless to the demons onslaught.
@Jeirien or having a human who can do all the 4 specialties, you can train your troops against it effectively, but the prev DK wanted him to learn magic so I guess that's not it?
Titties motivation men to do the impossible.
Also she seems really into him, I wonder if it was love at first sight?