Definitely Hermione, as she's always been by Harry's side in the original series and her knowledge would be crucial in time - travel scenarios. Ron is also likely to be there. Their trio is a strong bond. And then there's Hagrid. Harry might go back to have more interactions with Hagrid, maybe to learn more about his past or to prevent some bad things that happened to Hagrid in the original timeline.
Hermione is often one of the most common characters. Since she's Harry's best friend and very intelligent, she's likely to be involved in any time - traveling adventures. Another is Dumbledore. Harry might seek his guidance in the past, and Dumbledore's wisdom would be invaluable. Also, Sirius Black could be a character Harry interacts with, especially if he wants to change the events that led to Sirius' death.
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.
Ron is also a common character. He could provide the comic relief as well as be part of the adventure with Harry. Dumbledore might also show up as he is the wise mentor figure. Harry might seek his advice on how to handle the changes he wants to make in the past without disrupting the future too much.
Obviously, Harry Potter himself is the most common. Since it's about him being thrown back in time, he's the central character. Hermione Granger also often appears, as she's Harry's closest friend and her intelligence would be a great asset in the past. Ron Weasley might also be there, as part of the trio.
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.
Common characters he could meet include an au Ginny who is a powerful seer. There might be a Hagrid who is a tiny, timid man instead of the large, boisterous one we know. He could also encounter an au Sirius Black who is a Death Eater, which would be a shocking twist. Additionally, he may run into an au Professor McGonagall who is more of an artist than a Transfiguration teacher.
Well, in such fanfiction, Harry Potter could meet different versions of Dumbledore. In one universe, Dumbledore could be a dark wizard instead of the wise mentor we know. He may also come across alternate versions of Sirius Black. Maybe in some universes, Sirius is still in Azkaban for a crime he actually committed. Additionally, he could meet different versions of Luna Lovegood, some of whom might have no interest in the strange and wonderful things she loves in the original universe.
The most common characters are Harry, Hermione, and Ron as they are the main characters in the original series. However, Voldemort also shows up a lot in these fanfictions. In a time loop, the writers might explore different ways of defeating him or different aspects of his character. Additionally, Luna Lovegood can be a common character. Her unique perspective might help in understanding the time loop better. For example, she might have some off - the - wall theory that turns out to be the key to breaking the loop.
Harry himself is obviously a very popular character in these fanfictions as he is the one being sent back in time. But also, characters like Sirius Black are often popular. Fans like to explore how Sirius would react to Harry being in the past, and how their relationship could develop differently.
Definitely Harry himself. Since he is the one being sent back in time, he is central to the story. His actions and decisions in the past are what drive the plot forward.
He might meet versions of himself from different timelines or parallel universes. Also, there could be new and unique magical beings that the author creates just for the fanfiction. For example, a race of half - human, half - phoenix creatures.