Obviously, Harry Potter and Voldemort are the main characters. Harry, who makes the unexpected choice to join Voldemort. And Voldemort, who welcomes Harry into his ranks, perhaps with an ulterior motive.
Dumbledore would surely play a role too. He might be the one who tries to figure out how Harry was influenced to make such a decision. Dumbledore's wisdom and knowledge of the magical world would be crucial in trying to stop Harry from fully aligning with Voldemort and the dark side.
Maybe Harry joins Voldemort because he discovers some dark secrets about the so - called good side in the wizarding world. He might think that Voldemort's way of ruling could bring a new order that is more just in his eyes. However, he soon realizes that Voldemort's true nature is far more cruel than he expected, leading to a conflict within himself.
Harry Potter is central as the possessed. Voldemort is crucial as the possessor. Harry's friends are bound to be main characters. Hermione, with her intelligence, would be constantly observing Harry's strange behavior and coming up with theories. Ron, being Harry's loyal friend, would be by his side and ready to fight for him. Also, characters like Sirius Black, if he's still alive in the fanfiction's timeline, could be involved. He has a close connection to Harry and would surely want to help him get rid of Voldemort's control.
Snape could play a crucial role. Given his past with both Voldemort and his role at Hogwarts, he might be torn between his loyalty to Dumbledore's cause and his complex relationship with Harry. He could be the one who has to make a difficult decision when Harry starts to side with Voldemort. Additionally, Draco Malfoy could be involved. He might see Harry's change as an opportunity to gain more power within Voldemort's ranks, or he could be jealous of Harry's new - found position with the Dark Lord.
The Weasley family in general would be involved. Ginny, for instance, would be heartbroken seeing Harry on Voldemort's side. The family's loyalty to Harry would be put to the test, and they would be among the main characters as they try to either bring Harry back or fight against him if he goes too far with Voldemort.
Obviously, Harry Potter himself would be the central character. Then, Voldemort would be a major figure as he is the main representative of the Dark Side. Bellatrix Lestrange could also play an important role as she is one of Voldemort's most loyal followers. She might be the one to initially tempt Harry or guide him into the ways of the Dark Side.
Obviously, Harry Potter and Voldemort are the main characters. But there could also be other important characters like Hermione and Ron. Hermione might be trying to understand Harry's decision and Ron could be in shock or even feel betrayed at first. Dumbledore, even in spirit form perhaps, could also play a role as he was always a guiding figure for Harry and his presence could still influence Harry's actions in this strange new situation.
Hermione and Ron are also likely to be important characters. They would be there to support Harry, try to figure out what's going on, and probably try to save Harry from this dangerous situation. Hermione with her vast knowledge of magic might be trying to find a way to break the connection between Harry and Voldemort, while Ron would be there with his loyalty and courage to stand by Harry.
There could be his drill sergeant, a tough and no - nonsense character who has to whip Harry into shape. Maybe there's a fellow recruit who becomes Harry's best friend. They could support each other through the difficult training.
Obviously, Harry Potter and Voldemort would be the main characters. Since it's about an 'evil Harry Potter', he would be a central figure with a different personality than the canon Harry. He might be cruel, power - hungry and have a disregard for the rules of the wizarding world. Voldemort, on the other hand, would be the rescuer, perhaps seeing Harry as a tool or a kindred spirit in his evil plans.
The main antagonist could be Dumbledore. Since Harry has joined Voldemort, Dumbledore would see Harry as a threat to the good of the wizarding world. Dumbledore would try to bring Harry back to the right side, and they would end up in conflict.