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.
One of the characters most likely to be affected is Dumbledore. Harry going back in time could change Dumbledore's plans and strategies against the dark forces. Ron would also be affected as he is Harry's other best friend. Their friendship might develop differently if Harry changes things in the past. And then there's Hagrid. Harry might be able to help Hagrid in ways that he couldn't in the original timeline, perhaps by preventing Hagrid from getting into certain troubles.
His father's friends, the Marauders, are very likely to interact with Harry if he goes back in time. He could become a part of their group, learn their secrets, and even participate in some of their pranks. Hagrid is another character. Harry has always had a special bond with Hagrid, and going back in time, he could strengthen that bond even more. And of course, Snape. Harry could try to change their relationship from the start and understand Snape better.
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.
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.
Hermione would likely be there. She's Harry's best friend and would not let him be in juvie alone. She'd probably be trying to find a way to get him out legally, researching all the laws and loopholes. Ron might also show up, being another one of Harry's closest friends. He could be the more impulsive one, thinking about breaking Harry out rather than going through the proper channels. Dumbledore might also be involved in some way. He has always been a guiding figure for Harry, so he could be working behind the scenes to ensure Harry's safety and find a way to clear his name.
In 'Harry goes to Marauders time' fanfiction, Harry is the central figure. Alongside him, the Marauders are essential. James, the confident and sometimes arrogant young wizard, will be there. Sirius, the rebellious and loyal friend. Remus, the kind - hearted werewolf. And Peter, who might be on a different path in this alternate timeline. Lily, with her kind nature and powerful magic, will also be a prominent character. There could also be other Hogwarts professors from that time, like Dumbledore, who could play a role in Harry's journey in the past.
Obviously, Edward would be deeply impacted. He has dedicated himself to Bella, and her new evil nature would force him to reevaluate their relationship. Jacob, who has always been in the mix with Bella, would face a great deal of turmoil. And don't forget Carlisle and the rest of the Cullen family. They have accepted Bella into their family, and her going evil would be a major threat to their way of life and their values.
Hermione is another common character. Since she is always so intelligent and resourceful, Harry might turn to her for help in understanding the implications of time travel. They could work together to change certain events, like preventing the release of some dark magic that they later had to face. And of course, Ron would also be around. His loyalty and friendship would be a source of support for Harry as he navigates this new timeline.
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.
Neville Longbottom can also be a main character in some 'Harry Potter goes back in time' fanfictions. Given his growth and importance in the original series, in the fanfic, Harry might go back and help Neville become more confident earlier. This could have a significant impact on the fight against Voldemort as Neville could then be an even more powerful ally from an earlier time.
Obviously, Harry Potter and Clark Kent would interact a lot. They are both iconic characters in their own universes. Clark, with his superpowers, would be curious about Harry's magic, and Harry would be interested in Clark's abilities. Also, Hermione Granger, if she somehow also ends up in Smallville, would likely interact with Chloe Sullivan. They are both intelligent and inquisitive, so they could bond over solving mysteries or studying the differences between their two worlds.