Well, Harry Potter himself is surely one of the main characters. Since it's about him discovering magic early, he'll be at the center of the story.
I think Dumbledore could also be a main character. He might play a role in guiding Harry or covering up for his early use of magic. He would be aware of Harry's special situation and try to ensure his safety and development in the wizarding world.
Perhaps Hagrid could be a main character too. He has a special connection with Harry and might be the one who first notices Harry's early magic. He could be the one to introduce Harry to more about the wizarding world before he even gets to Hogwarts, like taking him to Diagon Alley secretly or showing him some magical beasts.