The main characters would be Harry Potter and the Queen. Harry, as the hero from the wizarding world, brings his magic, courage, and sense of justice. The Queen represents the Muggle royalty and all the power and traditions that come with it. There could also be some secondary characters from the wizarding world like Hermione and Ron, who might give advice to Harry on how to interact with the Muggle Queen.
There could be some of Harry's friends who might play a role. They could be the ones who encourage Harry to meet the Queen or who give him advice on how to behave in front of her.
It could be a power - hungry alien leader who wants to take over Hogwarts and use its magic for their own empire.
A dark wizard from the Harry Potter universe could be the main antagonist. Maybe a follower of Voldemort who escaped the final battle and found his way to Byakko's world. He sees the power of Byakko and wants to use it for his own evil purposes. He could try to enslave Byakko and use its power to take over both the wizarding world and Byakko's realm.
Most likely, Medusa would be the main antagonist. Her ability to turn people to stone makes her a very dangerous foe for Harry Potter and the other characters in the wizarding world.
It could be a dark fairy who wants to use the power of both the wizarding world and Pixie Hollow for her own evil purposes. She might try to steal the magic from Hogwarts and the pixie dust from Pixie Hollow to become all - powerful.
Maybe the main antagonist could be Ra's al Ghul. He's always interested in power, and if he got his hands on some magic from the Harry Potter universe, he could become an even greater threat. He could try to use the magic to bring about his vision of a 'perfect' world, which would be at odds with both Harry's and Nightwing's ideas of justice. So they'd have to join forces to stop him from achieving his goals.
I think it could also be Eminem. He might be the main character who is brought into the wizarding world and has to navigate through all the magic and mystery while still being his tough - talking self. He could bring his street - smart attitude to Hogwarts and change the way the students and teachers think about Muggles. For example, he might challenge the pure - blood ideology by showing that Muggles can have a strong and influential culture too. He could rap battles against some of the more snobbish pure - blood students and win them over with his skills.
It would probably be Harry Potter himself. He's from a world full of magic and mystery, and seeing him interact with the very real and regal world of the Queen would be fascinating.
Obviously, the Queen would be one of the main characters. And Harry Potter himself would probably be in it too, since it's a Harry Potter fanfiction.
Obviously, the Queen of England would be one of the main characters. Harry Potter himself would also be a key character as he is the central figure in the Harry Potter universe. Maybe Hermione could also be involved as she is very intelligent and could help the Queen understand the wizarding world better.