One common theme is the struggle between good and evil. The hero represents the good side, and the demon king the evil, and their conflict drives the plot. For example, in many novels, the hero tries to defeat the demon king to save the world.
Power and its consequences is also a theme. Both the hero and the demon king often have great power. The hero has to learn how to use it responsibly, and the demon king's power might be the cause of his downfall or transformation. For instance, a demon king's excessive thirst for power might lead to his overconfidence and eventual defeat by the hero who uses power more wisely.
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.
One common theme is the battle between good and evil. The hero represents good and the demon king represents evil, and they fight for the fate of the world.
Another theme is redemption or change. Sometimes, the demon king starts as an evil figure but through interactions with other characters, he may start to change his ways. For example, in some novels, the demon king may start to care about the well - being of his subjects more than just power. This can be seen in how he might make policies to improve the living conditions of the people in his domain. And also, the theme of mystery often exists. There are usually secrets about the demon king's past, or about the game world itself that gradually unfold as the story progresses.
Power is a common theme. As a great demon king, characters often have to deal with the acquisition and proper use of great power. For example, in 'Overlord', the main character has to figure out how to use his overwhelming power in the new world.
One popular 'demon king x hero light novel' is 'The Rising of the Shield Hero'. In this story, the hero is wrongly accused but still has to face various challenges including the threat from the demon forces. It has an interesting take on the relationship between the hero and the forces that seem like the 'demon king' side at times.
Well, in this kind of light novel, the demon king could be summoning the hero to challenge the traditional roles. Instead of fighting each other, they might work together against a corrupt system in their world. For example, there could be an overarching evil force that is oppressing both the demons and the humans, so the demon king realizes that the hero's power is needed to defeat it.
The theme of change is also prevalent. His entire lifestyle, values, and relationships change. He might go from being feared to being respected or befriended. There is also often a theme of secrecy. He has to keep his past a secret in some cases, which adds an element of suspense to the story. He may struggle with when and how to reveal his true identity, especially when he starts to care about the people in his new life.
Power struggle is a common theme. Usually, the demon king girl has to deal with various forces that either oppose or try to use her power. For example, in many stories, there are other kingdoms or powerful entities that see the demon king girl as a threat.
Power. Usually, the reincarnated demon king has some form of great power from their past life that they can tap into. This power often makes them stand out in the new world they are in.
Power management is a common theme. The demon king often has to figure out how to use his power effectively in a more practical sense, not just for destruction.