Since I haven't read this particular fanfiction, it's hard to say for sure. But it's likely to be the Black Queen herself. She would probably be the central figure around which the story revolves.
In fanfiction, the main character in 'the black queen harry potter fanfiction' could be the Black Queen. However, it might also be a new character who is closely associated with the Black Queen. For example, a young wizard or witch who discovers the secrets of the Black Queen and tries to either stop her or join her cause. Another possibility is that it could still be Harry Potter, but the Black Queen is a major new antagonist in his story.
I'm not sure as I haven't read it, but based on the title, it could be the 'black prince' himself.
I'm not sure specifically as there could be many different interpretations in fanfiction. It might be about a female character who is seen as the 'Black Queen' in the context of the Harry Potter universe. Maybe she has dark magic powers or is involved in some sort of power struggle within the wizarding world.
It could be an original character created specifically for the fanfiction. A young and beautiful queen of a small, yet prosperous kingdom, who has a strong will and a love for her people. She might be open - minded and curious about the wizarding world, which makes her a perfect match for Harry.
It might be an ancient magical being that has the form of a queen. She could have ties to the very origins of magic in the Harry Potter universe and decides to save Harry because she foresees his importance in the future of magic. For example, she could be a guardian of some sort of magic that is crucial for Harry to defeat Voldemort in the long run.
It could be the Queen of England, as in the real - world monarchy. In the fanfiction, she might be a figure with hidden knowledge of the wizarding world and decides to use her power to rescue Harry.
It could be Sirius Black's younger sister who was not mentioned in the original series but is created for the fanfiction. She might have similar rebellious traits as Sirius but also her own unique charm that attracts Harry.
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.
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.
Perhaps there's a member of the royal court who is secretly envious of Harry's power and tries to undermine him. This person could be plotting against Harry and trying to make the queen believe that he is a danger rather than an asset. They might spread false rumors or create false evidence to get rid of Harry.
It could be a dark wizard who wants to disrupt the new relationship between the wizarding world and the queen. This wizard might see Harry's position as a threat to his own plans of taking over the Muggle world starting from the monarchy.