[FairyKing]

I don't think it's actually accurate to say demons are always inherently evil. Even setting aside all the fiction that portrays demons as sympathetic, even in mythologically it's not that black-and-white. The original Greek word daemon that the modern word is descended from just meant any supernatural occurrence, even the actions of the gods themselves!

And even after the word demon got its evil associations in Judaism, Christianity and Islam, many spirits and deities from other religions got labeled as "demons" regardless of whether they were good, evil or something else entirely, either because it was just the closest translation someone could think of at the time, or as a way of saying "our God is the one REAL God, everyone else's gods are FAKE and EVIL!"

Alternatively, I wouldn't be surprised if the demons in Revice are more like "inner demons" or dark sides, the voice of your inner conflict or the part of yourself that might be considered evil but is still an essential part of you. And the idea that power can be found by reaching an agreement with your inner demon, owning it as part of you and drawing strength from it instead of trying to suppress it, would fit very well in Kamen Rider.