Obviously, Harry himself is the central character. Hermione would likely be involved as well. She could be the one who helps Harry research and understand his past - life memories. Maybe she finds ancient texts that give clues about what Harry is experiencing.
Ron would also be part of it. He might be initially skeptical about Harry's past - life claims but then come around to support him. Dumbledore could also play a crucial role. He might have known something about Harry's past lives all along and be guiding Harry in a more subtle way than he did in the original series.
Voldemort would still be a crucial character. Since Harry is redoing his life, he might try to defeat Voldemort in a more strategic and less dangerous way. Other characters like the Weasley family, who were always there for Harry, would be involved too. They could be part of Harry's new support system in his redone life.
Another interesting plotline could be that in his past life, Harry had a relationship with a character that is very different in his current life. This could create a lot of emotional turmoil and interesting character dynamics as he tries to reconcile his past feelings with his present situation.
Obviously, Harry himself is the central character. Hermione would probably be involved too. She might try to reason with Harry at first but then end up supporting him. Ron would also be there, likely being a bit more hesitant but still standing by his friends.
Obviously, Harry himself is the central character. Hermione and Ron would also likely be involved. They are Harry's best friends and usually play important roles in his adventures. Hermione with her intelligence might help Harry figure out strategies for the tournament tasks, and Ron with his loyalty would be there to support Harry.
Voldemort would also be a key character. His obsession with immortality and his actions that led to many deaths in the series are fundamental to the theme of life and death. In a crossover, he could be pitted against new threats or forces that have different views on death, which would make for an interesting story.
Obviously, Harry Potter himself is the main character as the whole concept is about him reliving his life. His friends like Ron Weasley and Hermione Granger are also likely to be main characters as they are an important part of Harry's life in the original story and will surely play a role in this new scenario.
Some of the teachers at Hogwarts could be involved as well. Professor McGonagall, for instance, might teach Harry some advanced Transfiguration magic. And Snape, before his death, could have left behind some notes or knowledge about Potions - related magic that Harry discovers and uses to learn more powerful potions and the magic associated with them.
In this fanfic, Harry is the main focus as he's the one sent to Azkaban. However, other characters would also be important. The guards at Azkaban, the Dementors, would be key figures as they would be the ones constantly menacing Harry. Then there are the Ministry of Magic officials who were involved in sending Harry there. Their motives and actions would be crucial elements in the story.
Hermione would definitely be involved. She's always been by Harry's side, and in this kind of fanfic, she would likely be helping Harry in his struggle, using her intelligence to figure out solutions to the problems he faces. Ron would also be there, providing his loyalty and courage in the fight. Dumbledore might make an appearance too, perhaps as a guiding spirit or in flashbacks, offering his wisdom to Harry as he battles towards salvation.
Hermione Granger is also a likely character. Her intelligence and love for knowledge could lead her to discover a way to bridge the past and the future. She might use her Time - Turner knowledge or find some new magical method. And she could play a key role in helping past characters adapt to the future or vice versa.