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.
Maybe Harry realizes he made some mistakes in his past and decides to go back in time to correct them. He could try to save his loved ones who died in the original story.
One interesting plotline could be Harry using his knowledge from his first life to prevent certain tragedies. For example, he might be able to save Sirius Black much earlier by being more cautious about the events at the Ministry of Magic. Another could be him changing his relationships with his classmates. He might befriend Draco Malfoy in a different way, perhaps by reaching out to him and understanding his family situation better, which could lead to a very different dynamic at Hogwarts.
Well, Lucius Malfoy could have a different role. If Harry redoes his life, he could try to turn Lucius against Voldemort by using his knowledge of Lucius's family and situation. This would change the power dynamics within the Death Eaters. Bellatrix Lestrange could also be affected. If Harry were to act differently towards her, perhaps by trying to capture her earlier or using her as a source of information, her role in Voldemort's plans would be altered. And of course, Ginny. Harry could start his relationship with Ginny in a more mature and different way, which would change their dynamic and her role in the overall story.
If Percy Jackson were to redo his life in fanfiction, it would be a wild ride. He might start by being more upfront with his mother about his strange experiences. He could then take steps to better prepare himself for the world of demigods. Perhaps he'd form alliances with other demigods sooner, and they could all work together to face the threats from the start. This could lead to a completely different power dynamic in the demigod world. He might also have a chance to explore his powers more deeply without the immediate pressure of constant battles and quests.
One interesting plot twist could be that Percy discovers a hidden power within himself much earlier. Since he already has the experience of his first life, he might be more in tune with his demigod abilities. Another twist could be that he decides to side with different gods in the power struggles, changing the whole political landscape of the demigod world.
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.
In the 'Harry Takes Control of His Life Fanfic', he could be facing challenges that force him to take charge. For example, a new dark threat emerges and he decides to form his own group to combat it instead of relying on the existing Order of the Phoenix. He might also take control of his own emotions, like dealing with his anger towards Snape in a more assertive and mature way.
Obviously, Harry would be the main character. He is the one taking control of his life. Hermione and Ron would most likely be important characters too as they are Harry's closest friends and would be involved in his journey. Dumbledore could also play a role, perhaps as a mentor figure that Harry starts to question and break away from as he takes more control.
In some fanfics, Harry Potter takes control of his life by defying the traditional paths set for him at Hogwarts. For example, he might choose to study magic in a more self - directed way, rather than following the standard curriculum. He could explore ancient and forbidden magics on his own terms, which gives him a sense of autonomy and control over his magical abilities and future.