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.
One possible fanfiction could be about Harry Potter accidentally stumbling upon a time - turning device in Hogwarts and ending up in the 1940s. There, he might meet the young versions of his professors, like Dumbledore, and get involved in the events that were shaping the wizarding world at that time, such as the rise of Grindelwald.
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.
There could be many 'Harry Potter is a fighter back in time' fanfics out there. You could also join some Harry Potter fan communities on social media platforms like Reddit or Facebook. The members there are often very knowledgeable about different fanfics and might be able to point you in the right direction. Another option is to use Google and type in the exact title or a close variation of it, along with 'Harry Potter fanfic' to see if it shows up.
Obviously, Harry Potter himself is a central character in such fanfiction. Then there's Hermione Granger. Her intelligence could play a big role if she also travels back in time. Another popular one is Severus Snape. His complex relationship with Harry and his past make him an interesting character to include in time - traveling scenarios.
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.
Often, it changes the original plot by allowing Harry to prevent certain deaths. For example, he might go back in time to save Sirius Black or Dumbledore. This alters the emotional landscape of the story significantly.
In some fanfictions, time - traveling magic might work through a powerful magical artifact. For example, a special Time - Turner that is more advanced than the one Hermione used. It could have the ability to transport the user to any point in time they desire. Another way could be through a spell that requires a great deal of magical power and concentration. The caster would have to visualize the exact time and place they want to go to.
One possible plot could be that Harry is sent back in time by some powerful magic or a time - turning device. Once in the past, he realizes that he has a mission to become a fighter for a cause that was lost in the future. He might start by training himself in the older, perhaps more dangerous forms of magic that were forgotten in his time. He then begins to form alliances with wizards and witches from the past who can help him in his fight. This could involve battles against dark wizards other than Voldemort, or even against the Ministry of Magic if they were corrupt at that time. The story could also explore how his actions in the past start to reshape the future he came from in unexpected ways.