Obviously, Harry Potter is the main character as he is the one trying to save his parents. His parents, James and Lily Potter, would also be important characters even though they are the ones being saved. There might also be other characters like Dumbledore. Dumbledore could play a role in guiding Harry in his mission, either by providing advice or by giving him some magical tool or knowledge that helps him in his quest to save his parents.
One possible plotline could be that Harry discovers a powerful time - traveling spell in his third year at Hogwarts. He decides to use it to go back in time and save his parents. Along the way, he has to face challenges like avoiding detection by the time - turner regulations and dealing with dark wizards who are also lurking in the past.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.
Yes, it is possible in fanfiction. Fanfiction writers often explore 'what - if' scenarios. They could imagine a scenario where Batman has some sort of precognitive ability that he didn't know about before. With this ability, he could foresee the attack on his parents and stop it. This would create a whole new narrative for Batman, where his origin story is completely rewritten and his relationships with other characters in the Batman universe would also be different.
It could be due to a sense of abandonment. Fanfic writers might explore the idea that Harry, growing up without his parents, feels resentful. He may see other kids with their families and, in his confusion and pain, start to hate his parents for leaving him alone in the world, even though it was not their choice.
At the start, Harry would be stunned. His whole life has been built on the idea of his parents being heroes. When he realizes they betrayed him, he would be heartbroken. He might go on a journey to find out the real motives behind their actions. Along the way, he could become bitter and vengeful. However, he might also find some inner strength and decide to rise above the betrayal. He could use it as a motivation to break free from the shadows of his parents and create his own path, but not without a great deal of emotional turmoil first.
One possible reason could be that in the fanfic, his parents might be depicted as having made some big mistakes in the past that directly affected Harry in a negative way. For example, they could have made a deal with a dark force that put Harry in danger later on.
Often, it's shown through his internal thoughts. The writer might describe Harry having angry and bitter thoughts about his parents when he's alone, like constantly thinking about how they left him to face so many difficulties. For example, he may think 'My parents left me to deal with Voldemort, they didn't care.'
Well, it could be that Harry meets his parents through some magical time - turner accident. Maybe he's on a mission in the past and stumbles upon them.