Filch could be a key character too. Harry going back in time might mean more opportunities to outwit Filch and his attempts to catch students misbehaving. This would add a lot of comical moments as Filch is always so serious about his job and Harry could use his time - traveling knowledge to drive Filch crazy with his pranks.
In this fanfiction, Harry might go back in time and play a prank on the Hogwarts ghosts. He could use his knowledge of the castle's secrets to set up situations where the ghosts are caught off - guard. For instance, he could create a fake treasure hunt that leads the ghosts on a wild goose chase all around the castle, and along the way, they encounter all sorts of unexpected and humorous obstacles.
Definitely Percy Jackson. He's one of the central characters in the 'Heroes of Olympus' series, so it's very likely he would be involved in a time - traveling adventure.
The main characters are Harry and Hermione, of course. Harry, the Boy Who Lived, with his Gryffindor courage and his special connection to Voldemort. And Hermione, the brilliant and studious witch, always full of knowledge and quick thinking.
Obviously, the Marauders themselves, which include James Potter, Sirius Black, Remus Lupin and Peter Pettigrew.
Obviously, Harry Potter himself is the main character as he is the one getting back at the Dursleys. The Dursleys - Uncle Vernon, Aunt Petunia, and Dudley - are also main characters as they are the targets of Harry's revenge.
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.
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.
Obviously, Harry and Teddy are the main characters. Harry is a well - known wizard from the original series, and Teddy is his godson.
They are likely to meet their future selves. This could be a very strange and enlightening encounter for both of them. They might also meet their descendants, like their children and grandchildren, which would be a heartwarming experience.
The other main characters could include the Hokage of that time, like Hiruzen Sarutobi. Naruto might interact with him to try and influence the decisions of the village. Also, members of the Uchiha clan such as Itachi would play significant roles as Naruto could try to change their fates as well.