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.
Dumbledore could be a significant character even after his death. His spirit or the memories he left behind could influence the events. Harry might be haunted by Dumbledore's teachings and have internal struggles because of it. Also, Bellatrix Lestrange could be there. She could be the one to initially welcome Harry into Voldemort's group, seeing him as a valuable asset. And she could also be a source of conflict for Harry as she has her own ideas about how things should be in Voldemort's regime.
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.
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.
Well, it could be that Evil Harry is actually a long - lost relative of Voldemort in this fanfiction. He discovers this connection and, feeling a sense of kinship with Voldemort's ideology of pure - blood supremacy, decides to join him. He then uses his skills at Hogwarts, like his knowledge of spells and potions, to enhance Voldemort's army. He might also be involved in turning some of his classmates to the dark side, creating a rift within Hogwarts between those who follow him and those who oppose Voldemort's new ally.
Hermione could be a main character. She might be trying to find a way to rescue Harry from the timeout situation. Ron could also be there, supporting Hermione in her efforts.
Hermione is often a very popular character. She can bring her intelligence and magic skills to help Harry in new and interesting ways in the fanfiction world. For instance, she might come up with a unique spell to counter Voldemort's dark magic.
In many of these fanfictions, the Marauders (James, Sirius, Remus, and Peter) are quite popular. They were friends of Harry's parents and their presence in a time - travel story can bring a sense of nostalgia and also add a lot of action and adventure. Bellatrix Lestrange can also be a prominent character on Voldemort's side. Her fanatical loyalty to Voldemort and her dark magic skills can make for some intense and exciting scenes, especially when she clashes with the heroes in different time periods.
Hermione is often a popular character. She might play a role in helping Harry understand the implications of Voldemort's reincarnation, using her vast knowledge of magic and history.
Snape could also be a character in such a fanfic. His complex relationship with both Harry and Voldemort makes him an interesting addition. He might be in a difficult position, having to keep his cover as a double - agent while also trying to find a way to save Harry without revealing his true intentions to Voldemort.
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.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.