In a lot of these fanfictions, Hermione is also a key character. She might be the one trying to figure out the Dark One's plans through her research and intelligence. And Ron could be there as Harry's loyal friend, helping him in the fight against the Dark One.
Obviously, the Dark One is a main character. Harry Potter himself also often plays a crucial role as he is usually the one who has to face the Dark One in some way.
In this fanfiction, the main characters could be quite diverse. Since it's about a 'dark heir', there would likely be a character who embodies this role. It could be a previously unheard - of character with a complex backstory related to a dark legacy. Harry Potter might be involved to oppose this dark heir or to try and understand their motives. His friends, like Hermione and Ron, could also be part of the story. Additionally, there could be other magical beings or characters from the 'Harry Potter' lore, such as members of the Order of the Phoenix or Death Eaters, who are drawn into the plot due to the emergence of the dark heir.
Well, Harry Potter is most likely one of the main characters as it's a Harry Potter fanfiction. He would probably be at the center of the new revelations and story developments.
It's possible that the main character is the 'dark child' of course. There could also be a mentor - like figure, maybe an older wizard who tries to guide the child towards the light. And it wouldn't be surprising if there were some antagonists as well, such as other dark wizards who want to use the child's power for their own evil purposes.
There could also be an original character created specifically for this fanfiction who becomes the Dark Phoenix. This character would then be a central focus, with their transformation and the resulting events driving the story forward. Their backstory, motives, and relationships with other characters would be important aspects of the narrative.
Most likely Harry Potter himself would be a main character since he is in the title as the 'king of dark creatures'.
I'm not familiar with the exact details of this fanfiction, but typically in a Harry Potter - based fanfiction with a 'rise of the dark angel' theme, Harry Potter himself would be a main character as he is the hero of the original story. The 'dark angel' would be another crucial character. There might also be other characters from Hogwarts like Dumbledore, who would oppose the rise of the dark angel. Maybe some new characters created specifically for this fanfiction would also play important roles, perhaps characters who are related to the origin or the power source of the dark angel.
Since I haven't actually read this particular fanfiction, it's hard to say for sure. But based on the title, I would assume the 'dark lady' is a main character, and perhaps there's a male character whose perspective the story is from. There could also be appearances from the classic Harry Potter characters like Harry, Hermione, and Ron as they interact with this new 'dark lady' character.
There could also be other characters from the Harry Potter universe like Hermione and Ron. They might be involved in helping Harry navigate this new world of being connected to the Charmed Ones. And from the Charmed side, there could be Leo, their Whitelighter, who might have some interesting interactions with the magical folk from Harry's world.
Harry Potter himself could be a main character. He would be the one to sense the danger of the dark side dance and try to oppose it. Hermione would also be important as she could use her vast knowledge of magic to research about the origin and nature of the dance. Ron could be there to provide the comical relief but also be a loyal friend in the fight against the dark influence of the dance.
Obviously, Harry Potter himself is often involved. He's the hero of the original series, so in fanfiction about a dark dragon, he'd be the one to step up and face the new threat. Hermione and Ron would likely be by his side too, as they are his closest friends.