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.
He could meet historical figures like Julius Caesar. Nero might have a conversation with him and learn about ancient Roman politics.
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, 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.
In this fanfiction, Nero might change historical events. For example, he could prevent certain disasters or alter the fates of key characters. Maybe he'll meet his ancestors and learn secrets about his family's past.
One interesting concept could be Nero's interactions with different historical figures. For example, how he might react when meeting a great philosopher or a powerful leader from the past. It could add a lot of depth to his character.
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 is the main character. He's the one going back in time. Then there are the Marauders - James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew. Lily Evans, Harry's mother, also often plays a significant role as Harry might interact with her in the past. Dumbledore could be another important character, as he might be aware of Harry's time - traveling or be involved in some way in the events that occur in the past.
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.
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.