Obviously, Harry Potter would be one of the main characters. Since it's related to the Dark Lord, he would also be a central figure. And probably Hermione and Ron would be main characters too as they are Harry's closest friends.
In addition to Harry, Ron, and Hermione, there might be some new characters introduced in this fanfiction. Maybe a powerful wizard or witch who has a connection to the Dark Lord's past and plays a crucial role in the story. There could also be a character who is an outcast from the wizarding community but holds the key to defeating the Dark Lord, or at least understanding his new plans. Also, Dumbledore, even if dead in the original series, might still have an influence through memories or prophecies, making him an important character in a way.
The Dark Lord himself is a main character. His actions and motives drive the story. Then there could be some Death Eaters who have more prominent roles in this fanfiction. They might be given more complex personalities and backstories. There could also be a character from the Ministry of Magic who is either trying to fight against the Dark Lord or is secretly in cahoots with him, adding more layers to the story.
One major plot twist could be that the Dark Lord has a hidden agenda that no one expected. Maybe he is using a false identity to deceive the wizarding world while secretly planning something even more devious.
The main character is James Potter. But depending on the fanfiction, other characters can become important. For example, if the story focuses on James' role in the wizarding government, there could be Ministry of Magic officials as major characters. Or if it's about his personal growth, his son Harry Potter (in a different timeline perhaps) could be a key character, and they could have interactions that shape both of their characters in new ways.
Obviously, the son of the Dark Lord would be a main character. He would be central to the story, with his actions and decisions driving the plot. Harry Potter himself might also be a key character. He could be a mentor figure to the son, guiding him away from the dark path. Hermione and Ron could also be involved, perhaps initially being suspicious of the son but later coming to understand him.
Obviously, Harry Potter himself is a main character, but in these stories where he's a dark lord, his friends like Hermione and Ron also play important roles. They could be either trying to stop him or reluctantly following him. Voldemort might also be a key figure, perhaps as a mentor or a rival to the new dark lord Harry.
Severus Snape's memories or a ghostly apparition of him could be a main character. Since he had such a complex relationship with the dark lord, he could offer insights to the daughter. Also, Ginny Weasley could be significant. She's a strong witch in her own right and could be a role model for the dark lord's daughter. And then there are the professors at Hogwarts like McGonagall who would have to deal with the situation of having the dark lord's daughter at school.
It could also be an existing character from the Harry Potter series like Hagrid. Given his love for all creatures, including some that are considered dangerous, he could be at the center of a story about the 'Lord of Dark Creatures'. He might be trying to protect these creatures from those who would use them for evil purposes or trying to rehabilitate them.
Given that it's a Harry Potter - themed fanfiction, Harry Potter would be a prime candidate for a main character. However, new characters might also be introduced in this fanfiction. Maybe there's a wizard or witch who has a special connection to this never - was castle. They could be guiding Harry and his friends through the mysteries of the castle, or perhaps they are the ones causing trouble within it. Hermione, with her vast knowledge of magic, would surely play an important role in exploring this strange new place as well.
Hermione could be one of the characters in the harem. Her intelligence would be an asset to Harry, even as a Dark Lord. She might be torn between her loyalty to the original Harry she knew and her feelings for this new, darker version. Ginny could also be part of it. Given her connection to Harry in the original story, her presence in the harem could add an interesting dynamic, especially if she tries to change Harry back to his old self.
Hermione would surely be one of the main characters interacting with Harry. She might try to bring him back from the dark side, constantly arguing with him about his choices and trying to remind him of their old friendship and the good they used to fight for.
In this fanfiction, Voldemort could still be a significant character. His relationship with the Grey Lord Harry Potter would be very different from the original. Instead of a clear - cut enemy, they might have a more nuanced relationship, with both having grey areas in their moral compasses.