Hmm... I have some questions about the mechanics of this world.
How common are crystals?
Like, in terms of usage by the public, are there cheaper/weaker crystals that are used for generating heat to cook food and such, or mostly just military usage?

Do crystals have inherent magic power?
I was going to have my character use a weapon with a slot for crystals that he would exchange to alter the effects/magics of the weapon. But from what I can gather, crystals do not have inherent magic power (I was going to have the crystals slowly drain over time, and then I would have to take them out and let them recharge). Since I was planning to make my character unable to use magic, having to rely on magical items.

Which brings me to my next question: Do magical items (inscribed items) have inherent magic power? 
For example, having clothes that never get dirty due to an inscription.

Another question I have: Would spirits/ghosts/fey, if they exist, be classified as a demon? Such as a fire spirit or Will-o'-the-wisps?
I may or may not be considering using a Kasa-obake as a weapon...

Are all demons evil?
I mean, are there neutral demons like... a demon-cow that grazes on grass?