@TheTanJar7 That is definitely a better theory than mine.
I was thinking he was actually born a demon without magic, and magic interferes with auras and the contracts the guild uses, thus why he can use them; and the demon king was raising him simply because he was unique. I'm kinda leaning towards the last Demon King being one of those nice characters, just doing his job; while his son is the more traditional hates all humans because DKs.
A generic Mc, overpowered. Shows no conviction towards the treatment he received in the demon army. And no malice or development towards the village. And for that I’m out.
Was he really a soldier in an army? He should at least know the basics of the different kinds of weapons. It turns out that what was meant by "Demon King's Aide" is "Demon King's Bed Warmer."