In a fanfic, the bug demon mate could be a part of an ancient prophecy in the Harry Potter universe. It might be foretold that a bug demon and a wizard (Harry) would come together to bring balance. The bug demon's nature could be a contrast to the more civilized wizarding society. It could introduce a wilder, more unpredictable side of magic. Also, it could have its own set of enemies within the magical world, which could draw Harry into new conflicts and adventures.
Perhaps she could be a part of a hidden underworld within the wizarding world that Harry and his friends are unaware of. The female demon could be a key to unlocking secrets about this underworld, like ancient spells or hidden treasures. Her presence could stir up trouble among the wizarding community who are either scared of her or want to use her for their own gain.
A Wendigo could be a dark and mysterious addition. It could be part of a new threat that emerges from the Forbidden Forest. The magical creatures in the forest might be afraid of it, and it could start preying on students who wander too far. Harry and his friends would need to use spells like the Patronus to protect themselves.
A fanfic with 'Harry Potter has a bug demon mate' could start with Harry discovering this strange being. The bug demon mate might look quite unusual, with features that are part bug and part something else. At first, Harry might be a bit wary, but as they interact more, they realize they can work together. The bug demon could have the ability to communicate with other bugs, which could be used for gathering information. For instance, it could send out bugs to spy on the Death Eaters. Their friendship could be a key element in the fanfic, and the bug demon's loyalty to Harry could be unwavering as they face various challenges together in the wizarding world.
It would be a very difficult fit. Since the wizarding world has its own norms like monogamous relationships in most cases (like the marriages we see in the series). If one were to introduce concubines, it would have to be in an alternate universe where the social and moral values are completely different. Maybe in a fanfic where the wizarding world is modeled after a more ancient and permissive society, but this would be a huge departure from the original feel of the Potterverse.
Mileena could be a foreign exchange student of sorts from a different magical realm. Her unique appearance and combat skills would make her stand out at Hogwarts. She might have trouble adjusting to the more academic and rule - based magic of the wizarding world compared to her own more instinctive and combat - focused magic.
He could fit in as a unique hero. His magic could be seen as a special power like the others in DC. For instance, he could use his Patronus charm to fight off fear - inducing villains.
Alucard could be a part of an ancient prophecy in the Harry Potter fanfiction. His arrival in the wizarding world could be foretold. He might be seen as a savior or a bringer of chaos. His powers could be seen as a necessary evil to defeat a greater threat, like a new and more powerful form of Voldemort. He would bring his own brand of violence and mystery to the relatively more structured world of Harry Potter, making for a very exciting and unpredictable story.
Hawkgirl could be a new kind of magical creature. Her ability to fly with her wings would be a unique form of magic in the wizarding world. She might be a protector of some hidden magical place in Hogwarts or the Forbidden Forest, like a guardian of an ancient magic that Voldemort is after.
They could be exchange students from a different magical school. Their presence would disrupt the normal routine at Hogwarts. For instance, they might introduce new spells or magical items from their own school. Their sense of humor would also be a big change. They could make fun of some of the old - fashioned Hogwarts traditions, but also learn to respect them in the end.
The OC Lightning Demon might also be a character that is initially misunderstood. It could be that it is actually trying to protect something in the Inuyasha universe, like a sacred place or a certain group of beings. So it uses its powers defensively, but Inuyasha and the gang mistake it for aggression at first.