First, study the existing world of Makai Ouji. Know the rules about devils, their powers, and how realists interact with them. For example, if devils have certain hierarchies and magic systems, your OC should fit into that framework. If your OC is a devil, they should have powers that are consistent with what we know about devil powers in the series.
One idea could be an OC who is a powerful mage from a forgotten lineage. They could be brought into the world of Makai Ouji to help balance the power between the devils and the realists. Maybe they have a special ability that can temporarily nullify devil powers, which makes them both a threat and a valuable asset.
The significance could lie in the contrast presented. 'Devils' often represent chaos or immorality in literature, while a'realist' is more grounded. In an old love story, this could mean that the love is between two very different characters. The story might explore how they overcome their differences or how their relationship is affected by their opposing characteristics. It could also be about how love can exist even in the face of such extreme differences.
One idea could be a crossover where a character from 'Harry Potter' accidentally stumbles into the world of 'Devils and Realist'. For example, Harry Potter himself could find himself in the midst of the political intrigue in the 'Devils and Realist' world, having to use his magic in new and unexpected ways to survive and figure out how to get back home. Maybe he forms an unlikely alliance with one of the main characters from 'Devils and Realist' who is intrigued by his magic powers.
Well, the main characters would be the devil - like figure and the realist. The devil - like character could be someone with a sense of rebellion, perhaps with a dark past or a different set of values compared to the realist. The realist, on the other hand, might be someone who values stability, honesty and hard work. There could also be some secondary characters, like a wise old mentor who tries to guide them in their relationship or a jealous rival who tries to come between them in this old love story.
The magic in 'Harry Potter' is mainly based on spells, potions, and magical artifacts. In 'Devils and Realist', there are different types of powers related to demons and the political power plays. In a fanfic, one could combine them by having a spell that can summon or control a demon from 'Devils and Realist'. For example, a powerful wizard in the 'Harry Potter' world discovers an ancient spell that links to the demon realm in 'Devils and Realist' and starts to experiment with it, but quickly realizes the danger as the demons have their own motives and power hierarchies.