Typically, the characters that drive the most interesting parts of the original story will be in the fanfiction. So if there was a group of adventurers in the original, they would probably be the main characters in the fanfiction too. Their personalities might be explored more deeply, or they might be put in different situations compared to the original.
It depends on the source material it's based on. But usually, the main characters from the original work will be prominent. For instance, if it's related to a fantasy series, the hero and the main antagonist from that series would likely be main characters in the fanfiction as well.
Harry Potter himself is usually a main character. After all, it's a Harry fanfic. Hermione and Ron are also likely to be important characters as they are Harry's closest friends in the original series.
Often, a character named Sakura is the main focus. She could be a young girl with special powers or abilities related to the sakura, like being able to control the growth of sakura trees or having some sort of sakura - based magic.
Sometimes, there could be historical characters as main characters too. For instance, if Nero travels back to the Roman Empire, Caesar or Cleopatra could be important characters. They would interact with Nero, and their different personalities and goals would create interesting storylines. Nero might have to work with them or against them depending on the plot.
Well, of course, Iron Man is often involved. His intelligence and technological know - how can be crucial in figuring out the time travel mechanics. Captain America is also a staple as his leadership and sense of right and wrong guide the team during their time - traveling adventures.
Obviously, Naruto is the central character. But often, his friends like Sasuke, Sakura, and Kakashi are also involved. Sasuke might be there to have some sort of reunion or conflict with Naruto. Sakura could be the one to heal Naruto if he comes back injured. And Kakashi, as Naruto's former teacher, would be there to welcome him back or assess his new abilities.
Obviously, Harry Potter is the main character. He is the one being sent back in time after all. Dumbledore often plays a significant role too. Since he was such an important figure in Harry's life, he might be the one to guide Harry in the new timeline. Hermione and Ron are also likely to be important. They were Harry's closest friends, and in the time - travel scenario, Harry might try to involve them in his new plans early on.
Most likely Bella and Edward would still be main characters. Since it's a Twilight - based fanfic, their relationship is often central to the story.
Well, it might have some exciting plot twists that deviate from the original. It could be that a character who was a minor in the original becomes a central figure, leading to a whole new set of adventures and relationships.
Obviously, Naruto himself is the main character as he is the one sent back in time. Sasuke is also often involved as Naruto's relationship with him is a central part of the Naruto story. Kakashi could be involved too, as Naruto might seek his help or try to influence his training methods. Also, the various Hokages might be important characters as Naruto could interact with them in the past to change the course of events.
The main characters are Sam and Dean Winchester, of course. But in some fanfictions, Bobby Singer might also be involved. Since he was like a father figure to them, he could be the one guiding them through the time - travel process, or he could be there in the past waiting for them. Also, the characters of their parents, Mary and John Winchester, might show up if they go back to a time when their parents were alive.