Besides Harry, Sirius Black could be a significant character in this fanfiction. Even though he's on the run in the original story, in this scenario, he might find a way to secretly look after Harry. He has a strong connection to Harry as his godfather. Also, Ginny Weasley could play a role. She could be like an older sister figure to the deaged Harry. She might share her toys or tell him stories about the Weasley family. And of course, the teachers at Hogwarts like McGonagall would also be involved, making sure Harry is following the rules and learning at an appropriate pace for a three - year - old.
Hermione and Ron would most likely be main characters. They are Harry's closest friends in the original story. Since Harry is deaged, they would play important roles in taking care of him or guiding him in some way.
Snape could also be part of it. He might catch Harry in a rather awkward moment when he's trying to hold it in and give him a hard time. Or, on the other hand, he could surprisingly offer some help in a very Snape - like way.
Hermione Granger is often involved. She's Harry's best friend and is extremely intelligent. Her knowledge of spells and magic history can be crucial in helping Harry prove his innocence. She might research old laws or spells that can be used as evidence.
Hermione would probably be one of the main characters. She's smart and resourceful, so she could help Harry with the administrative side of running the orphanage, like handling finances and paperwork. Ron might also be involved, perhaps teaching the orphans some practical magic skills. And of course, there could be some of the orphans themselves as main characters, like a young girl with a special magical ability that Harry discovers.
Hermione could be one. She might follow Harry to the island to help him out. Another could be Ron, his loyal friend. Or it could be some new characters created specifically for this fanfiction, like a wise old wizard who has been living on the island for centuries.
The Weasley twins could be part of it too. Suppose Harry yells in excitement or anger about a new prank idea that the twins have come up with that might be a bit too dangerous. They could either encourage his yelling or try to tone it down, depending on their mood. And Malfoy might overhear Harry's yell and use it as an opportunity to taunt him, adding more conflict to the fanfiction.
Ron could also be a main character. He might be a bit scared or jealous of Harry's new - found ability. But in the end, he stands by Harry. Maybe there's a scene where Ron has to overcome his fear of snakes when they are in a dangerous situation that requires Harry to talk to a snake for help.
Hermione is often involved. Since she's Harry's best friend, she might be the one trying to prevent his death in some fanfictions. Or she could be the one who has to carry on his legacy after he dies.
Hermione Granger would still be one of the main characters. She would be by Harry's side, using her intelligence to help him understand his new powers and the implications of being the son of the 'Big Three'. She might also be researching ancient texts to find any references to such a situation in wizarding history.
Dumbledore could be involved too. He often has a deeper understanding of things in the wizarding world. He might be the one who arranges the heritage test for Harry, or at least be aware of it and offer his wisdom and guidance. Also, some of Harry's professors could be involved. For example, Professor McGonagall might be curious about the results and how it could affect Harry's performance in school. And, there could be some characters from Harry's family's past that make an appearance, either as memories or some sort of spectral form during the test.
Hermione would probably be involved. She's always by Harry's side, and she might be researching ways to help Harry tame the basilisk. Maybe she finds some useful spells or information in the library that gives Harry an edge.