What are the common themes in 'demon king and hero light novel'?
Common themes include good vs. evil, of course. But it's not always so black and white. There can be moral ambiguity. Another theme is the idea of fate. Are the hero and the demon king fated to fight? And how do they deal with that? Also, the concept of power corruption is often explored. The demon king may be corrupted by power, or the hero may face the temptation of power as he gains more strength. And the theme of perseverance is essential, as both sides usually have to keep going in the face of great difficulties.
A prevalent theme is redemption. Sometimes, the demon king may not be entirely evil, and there could be a chance for him to be redeemed, or the hero may have to struggle with his own dark side. Also, friendship and loyalty play a part. The hero often has comrades who support him, and the demon king may have his own followers. Their relationships within their respective groups are important aspects. In addition, the theme of sacrifice is often seen. The hero may have to sacrifice something precious to defeat the demon king or vice versa.