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.
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 other students at Hogwarts would also be affected. For example, Neville, who looks up to Harry. If Harry ignores everyone, Neville might feel lost. Ginny, who has a special connection with Harry, would be deeply hurt. And the teachers like Snape, who has a complex relationship with Harry, would also be impacted. Snape might be secretly glad at first, but then realize that Harry's isolation could be a sign of a bigger problem in the wizarding world.
Harry Potter for sure. And then, there could be a young local witch or wizard who becomes Harry's friend and helps him understand the North better. Dumbledore might also play a role, perhaps he sends Harry on this mission to the North in the first place. Additionally, there could be some antagonists, like a group of dark wizards who are causing trouble in the North and are opposed to the changes Harry is trying to bring.
Obviously, Harry's closest friends like Hermione and Ron. They would be devastated. Hermione would probably be racked with guilt for not being able to prevent it. Ron would be angry and at the same time lost without his best friend.
There could also be some minor characters that become more important in this fanfiction. For example, Harry's parents, James and Lily. Since Harry is changing the past, they might have more of a role to play. Or perhaps characters like Sirius Black or Remus Lupin could be involved in different ways due to the changes Harry makes in the past.
Perhaps Harry discovers some long - hidden family secrets that make him believe that the so - called 'good side' has been wrong all along. He might find out that his family has a connection to the Dark Arts that was misrepresented by Dumbledore and others. This newfound knowledge could cause him to change sides and try to rewrite the wizarding world's view of good and evil.
Definitely Hermione. Since she's Harry's closest friend and very intelligent, she would be deeply involved in any changes Harry tries to make. Also, Snape. Harry's actions in the past could completely change his relationship with Snape. And of course, Voldemort. If Harry goes back far enough, he could potentially stop Voldemort's transformation into the evil wizard he became.
Another plot could involve Harry being influenced by a family connection he discovers to a Death Eater. This newfound relation makes him reevaluate his beliefs and he starts to see the Death Eaters as more than just pure evil. He might think that by joining them, he can reform them from the inside.
The younger students at Hogwarts are quite likely to read Harry Potter fanfiction. They are at an age where they are really into the wizarding world and would be curious about different interpretations. Students like Colin Creevey, who is a big fan of Harry, might be eager to read fan - created stories about his hero. Also, the members of Dumbledore's Army, such as Luna Lovegood and Neville Longbottom, seem like the types who would be open to exploring fanfiction.
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.