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.
The main character is Harry Potter in his new role as a dark lord. Hermione might be a character who tries to use her intelligence to understand what made Harry turn and find a way to reverse it. Ron could be torn between his loyalty to Harry and his sense of right and wrong. Additionally, characters like Snape might have a different role too. He could be secretly working against Harry if he still believes in the side of good, or he could be one of the few who understand Harry's motives and support him, depending on how the story is written.
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.
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.
Apart from the dark lord, there are often heroes in 'dark lord stories'. These heroes are brave and determined to stop the dark lord's evil plans. Sometimes there are also side characters like the dark lord's minions or wise old sages who give advice to the heroes.
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.
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.
The teachers at Hogwarts, like Dumbledore (if he were still alive in the fanfic), would be deeply concerned. Dumbledore always saw great potential in Harry for good. He would probably try to understand what led Harry to this path and would use all his wisdom and magic to try and turn Harry back. The other teachers would also be on high alert, fearing for the safety of the students and the future of the wizarding world.
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.
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.
Obviously, Harry Potter is the main character. His journey as he becomes Lord Potter and how it changes his life is the central focus.
Obviously, Harry Potter would be one of the main characters. He would be the one most likely to interact with the concept of the 'Lord of Lightning'.