Obviously, Harry Potter himself is the central character as the title implies. Voldemort is also often involved as the main figure on the 'dark side' that Harry might switch to or interact with differently. Then there's Snape. His complex relationship with Harry could be a key factor in the story. In some fanfictions, he might be the one who either tempts Harry to switch sides or tries to prevent it depending on his own motives.
One interesting scenario could be Harry realizing that the so - called 'dark side' has been misrepresented. Maybe he discovers that the Death Eaters have some valid grievances against the Ministry's corruption, which makes him question his allegiance. Another could be that he's influenced by a powerful dark magic artifact that makes him see things from Voldemort's perspective, at least temporarily.
Another plotline could involve Harry being betrayed by the Order of the Phoenix. Feeling hurt and lost, he decides to side with Voldemort out of revenge. However, as he gets closer to the dark side, he realizes that Voldemort's true nature is far more evil than he thought and has a change of heart.
Obviously, Harry Potter himself is the central character as the story revolves around his switching sides. Hermione and Ron also play significant roles. When Harry starts to show signs of siding with the dark side, Hermione might try to use her intelligence to figure out what's wrong and bring him back. Ron, on the other hand, could be more impulsive in his reactions, either trying to fight Harry if he becomes a threat or trying to persuade him to return to the good side.
One way Hermione could switch sides in fanfiction is through a major misunderstanding. For example, if the wizarding world turned against her for some false accusation, she might feel betrayed and be pushed towards the 'dark side'. Maybe she discovers that the so - called 'good guys' have been hiding some dark secrets that go against her moral code.
In a 'Draco switches sides' fanfiction, it might start with Draco being disillusioned with his family's long - standing loyalty to Voldemort. His experiences at Hogwarts, especially his interactions with some of the more morally upright students like Hermione or Harry, could gradually nudge him towards the other side. He might first resist the idea but then, after a major event, such as seeing an innocent person being brutally attacked by Death Eaters, he decides to switch sides and use his position within the Death Eaters to undermine them from within.
Harry Potter would surely be one of the key characters. His relationship with Draco, which has always been a mix of rivalry and grudging respect, could play a huge role. As Draco switches sides, Harry would be initially suspicious but then come to accept and work with him. Hermione Granger is also likely to be important. Her intelligence and strong moral compass could influence Draco further and she could be the one to first recognize his change of heart.
Perhaps in this fanfiction, Harry discovers some dark secret about the Order of the Phoenix or Dumbledore that makes him lose faith in the side he has always been on. This, combined with Voldemort's manipulation, could lead him to side with the Dark Lord. For example, if he finds out that Dumbledore has been using him all along without his full knowledge, he might be so disillusioned that he turns to Voldemort as an alternative.
Obviously, Harry Potter himself is the central character in this type of fanfiction. Then there are often characters like Voldemort or his Death Eaters, as they are the ones Harry would be changing sides to. Bellatrix Lestrange could be a key figure in trying to persuade Harry or manipulate him into joining their side.
The Death Eaters themselves would be affected in an interesting way. Some might welcome Harry with open arms, but others might be suspicious of his sudden change of sides. They might fear that it's a trick by the Order. And within Hogwarts, the students would be in turmoil. Those who idolized Harry would be heartbroken and confused, and there could be a split among the student body depending on who believes Harry's new stance.