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.
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.
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 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.
Hermione Granger is also a common character. Given her love for knowledge and her skills in magic, she would likely be researching the best spells and magical theories to make the terraforming successful. She might be the one to find ancient spells that are crucial for the process.
In many expelled Harry Potter fanfictions, the Weasley family as a whole gets involved. Molly Weasley would be worried sick about Harry and would be cooking up plans to get him back. Fred and George might use their prank - making skills to expose the truth behind the expulsion. Ginny, being Harry's love interest in the original series, would be actively involved in trying to clear his name. And don't forget about the Hogwarts professors. McGonagall would probably be torn between following the rules and believing in Harry.
Harry Potter himself is often a central character. He might be shopping for new school supplies at Flourish and Blotts or looking for a new broomstick at Quality Quidditch Supplies. His friends, like Hermione and Ron, usually accompany him. Hermione might be more interested in books and rare magical artifacts, while Ron would be eyeing the latest Quidditch gear.
Mrs. Weasley is a very likely character. She's an expert at cooking and baking in the normal wizarding world, so in fanfiction about magical bakes, she could be teaching the younger characters how to make special bakes. Hagrid might also be involved. He could bring in some really strange ingredients from the Forbidden Forest, like giant acorns, to be used in the bakes.
Hermione Granger is often involved. In some fanfictions, she discovers hidden conspiracies within the Hogwarts library. For example, she might find a book that reveals a plot by some of the older pure - blood families to keep certain magical knowledge from half - bloods and Muggle - borns.
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.
Hermione is often involved. She might be the one helping Harry with the spells or potions to change his identity, like brewing the Polyjuice Potion correctly. Ron could also be involved, perhaps providing moral support or getting into trouble while trying to keep Harry's new identity a secret.