Most likely Harry Potter himself would be a main character since he is in the title as the 'king of dark creatures'.
There could also be some dark creatures that are important characters. For instance, a powerful werewolf or a vampire who either support Harry in his new role or oppose him. These dark creatures would add an interesting dynamic to the story as Harry has to interact with them on a different level now that he is their 'king'.
In addition to Harry, his friends like Hermione and Ron could also be main characters. They would probably play important roles in Harry's journey as the king of dark creatures. Maybe they would be his advisors or the ones who help him face the challenges that come with this new role.
It could be about Harry Potter having some sort of connection or role as the 'king' of dark creatures in a fan - created story. Maybe he discovers a hidden power within himself that gives him control over these usually feared beings.
The Dementors are quite popular. Their ability to suck out happiness and their overall spooky nature make them fascinating to write about. Werewolves are also common, especially with the whole Remus Lupin storyline in the original books. And the Basilisk is often used in fanfiction, with its deadly stare and connection to Salazar Slytherin.
Werewolves are also a popular dark creature in fanfiction. Writers may explore their relationships with other characters in more depth, like how they interact with Hogwarts students. For example, a werewolf character could be a misunderstood outcast trying to find his place.
There could also be a group of pure - blood wizards who look down on Harry's connection with magical creatures. They see it as beneath them and try to undermine his position as the king of magical creatures. They might spread false rumors about him and try to turn other wizards against him.
Most likely Harry Potter himself would be one of the main characters involved in creating creatures. After all, it's in the title. Hermione might also be involved, with her vast knowledge of magic and creatures, she could contribute a great deal to the process.
A less obvious candidate could be a corrupted Dumbledore. Imagine if he was swayed by the allure of absolute power and became the dark king. It would be a huge twist as he is seen as a symbol of good in the original books.
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.
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.
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.