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.
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.
In fanfiction, if Harry goes back in time to the god, he might gain some extraordinary powers. Maybe the god would bestow upon him the ability to control magic in ways he never could before. It could also lead to him changing the course of history, for better or worse.
Well, in some fanfictions, Harry goes back in time to fix his relationships. He might go back to be a better friend to Ron and Hermione from the start. Also, he could use his knowledge of future magical items to gain an edge. Like, he could get his hands on some powerful spells or artifacts before Voldemort does. And sometimes, he goes back to change the Hogwarts curriculum, making it more practical and useful for the coming battles.
In some fanfictions, Harry goes back in time and decides to train himself better. He could seek out different magical knowledge or combat skills that he didn't have the first time around, which then changes his future interactions and battles. For example, he might learn ancient spells that give him an edge against Voldemort.
One great fanfiction with this plot is 'Time's Redemption'. In it, Harry uses a powerful time - turning spell he discovers in an ancient book. He has to face many challenges in the past, like avoiding detection from the Ministry's time - turner regulations. Another good one is 'Back to Save Sirius'. Here, Harry teams up with Hermione and they use a secret time - travel device left by Dumbledore.
Well, they could meet younger versions of themselves and their friends. It would be really interesting to see how they interact. They might also try to give some hints or advice to their younger selves, but they have to be careful not to disrupt the timeline too much. And they could discover secrets that were long forgotten in the present time.
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.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
Well, a popular 'dark harry goes back in time fanfiction' is 'The Darkened Past Redone'. It's really interesting as it shows how Dark Harry's different perspective from his time - traveling affects the whole wizarding world's history. He makes some unexpected alliances and changes the course of many events. There are also stories like 'Time - Bent Dark Harry' which delves into his internal struggle between his dark self and the potential for change as he goes back in time.