Hedwig could be one. She might fly off to get help, like to Hermione or Ron. They could then come up with a plan to rescue Harry from the Dursleys.
Dumbledore might sense something is wrong through his magical connections. He could send someone like Hagrid to the Dursleys' to check on Harry and put a stop to Uncle Vernon's actions.
Maybe it shows how Harry endures the abuse from Uncle Vernon. It could explore Harry's feelings of helplessness and his thoughts during those tough times.
Obviously, Harry Vernon is a main character. But often, there are other characters created specifically for the fanfic or borrowed from the original source material if it's related to a known franchise. For example, if it's in a magical - themed fanfic, there could be a wise old wizard who guides Harry Vernon on his journey. Or a young sidekick who accompanies him in his adventures.
In this kind of fanfiction, perhaps Harry, out of fear or some form of coercion, starts to obey Uncle Vernon's every order. Maybe Vernon uses threats about expelling Harry from the house or withholding food to make him submit. This could lead to a very different dynamic in the Dursley household, with Harry being more submissive and not showing his usual defiance.
Sometimes, new original characters are created in fanfiction to help Harry. These could be students from other wizarding schools who have unique skills. For example, a student from a school in a far - off land who has mastered a type of magic that is unknown in Hogwarts and uses it to help Harry in his fight against the dark forces.
Obviously, Harry himself. And probably the doctor or healer who is doing the check - up.
One possible motive could be his intense hatred for the wizarding world. Since Harry represents that world to him, he might murder someone he believes is part of the magic that he despises so much. Another motive could be fear. If he feels threatened by a magical being or situation, in his panicked state, he could resort to murder.
Petunia could be a great addition. Her relationship with both Harry and Vernon adds a lot of complexity. She could be the mediator or the one causing more trouble between them, depending on the plot.
Hermione could be a great help. She is very intelligent and resourceful. She could help Harry find evidence against Snape. For instance, if Snape is suspected of cheating in grading, Hermione could use her knowledge of Hogwarts' grading system and records to dig up proof.
In some fanfiction, Uncle Vernon develops by becoming more accepting. For instance, he may start to see Harry not just as a freak but as a person with unique abilities that can be useful. He might begin to tolerate the wizarding world a bit more, especially if he realizes that Harry can use his magic to protect the family from certain threats.
One character could be Harry, of course. The other main character could be a singer named Ava who is performing in Vegas. She's got a big heart and a wild side. There could also be a Vegas local named Jack who befriends Harry and kind of guides him through the process of getting married in Vegas, giving him local insights.