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.
Voldemort could also play an important role. Since Harry is now the dark prince, there could be a complex relationship between them. Voldemort might see Harry as a potential ally or a threat, and their interactions would surely be a major aspect of the fanfiction, along with how Harry navigates this new relationship with the most feared dark wizard.
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.
Since it's a fanfiction, the main characters could vary greatly. It might introduce new original characters who are somehow related to the dark phoenix concept. However, it's also likely that popular characters from the original series like Ron would be involved. He could be by Harry's side as they both encounter the challenges brought by the dark phoenix rising. It could also involve some of the Hogwarts professors, like Dumbledore in spirit (if it's set after his death) guiding the students to face this new and dangerous situation.
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.
The characters in the 'dark trio' in the fanfiction are likely to be those who have a propensity for the dark arts or have some sort of malicious intent. It could be that the author takes minor characters from the original series and gives them more prominent roles within this dark context. For example, Crabbe and Goyle could be developed further into more menacing characters, or perhaps some of the Death Eaters that were not as well - explored in the original books could be part of this trio.