There could be an old hermit wizard who lives deep in the forest. He has been there for ages and knows all the secrets of the forest. He takes Harry under his wing and trains him in a different kind of magic that is more in tune with nature. The wizard could have a pet phoenix that also plays a role in Harry's growth, like giving him advice or helping him in difficult situations.
A group of magical animals like a family of foxes with special powers could be important characters. The foxes might be able to communicate with Harry in their own way and lead him to hidden places in the forest where he can find powerful magical items or learn new spells. They could also protect Harry from the threats that lurk in the forest, like dark spirits or other dangerous magical creatures.
A forest nymph could be important. She might guide Harry through the forest, teaching him about the plants and their magical properties.
There could be the caretaker of the orphanage. This person might be a strict or kind figure who has a significant influence on Harry's early life. They could be either a source of support or a constant obstacle for Harry.
Definitely Ginny. Since she's Harry's love interest in the original series, in the fanfiction about them growing up, she has a big role. She could be shown as a strong, independent witch who is equal to Harry in many ways. Draco Malfoy is also a character that features a lot. Sometimes, fanfiction explores his redemption arc further as he and Harry have to interact in the post - war wizarding world.
Hermione could be another main character. Maybe she discovers what's happening to Harry and the author and tries to find a way to reverse it or at least understand it better. She could use her vast knowledge of magic to research the phenomenon.
In this fanfic, perhaps Harry grows up isolated in the forest and has to rely on his own instincts and the magic within himself. He could stumble upon an old, hidden wizarding settlement in the forest that has been long forgotten. There, he finds ancient spells and artifacts that not only help him grow as a wizard but also give him a unique connection to the past of the wizarding world. This experience in the forest would shape him into a different kind of wizard compared to what we see in the original series.
Obviously, the illithid would be a main character. It could be a mysterious and powerful being that Harry encounters. Also, Hermione and Ron might still be important characters. They could be either suspicious of the illithid or try to help Harry understand it better. Dumbledore might also play a role, perhaps guiding Harry in his interactions with this strange new creature.
In addition to the trio, there could be characters like Professor McGonagall. She would surely notice the changes in Harry and might try to guide him. Also, the ghosts of the founders themselves could be considered main characters in a sense. Since Harry is possessed by them, they would have a significant influence on the story, perhaps communicating with Harry or causing events to unfold around him.
Besides Harry, there could be a character who is a descendant of an ancient elemental magic family. This character might initially be a rival to Harry in mastering the magic but later becomes an ally. Also, Dumbledore could play an important role. He might be secretly guiding Harry in his journey of learning elemental magic, using his own deep knowledge of the wizarding world's secrets.
Hermione would probably be involved. She could help Harry research about the island, like its history and any magical properties it might have.
Hermione would surely be one. She's smart and would be great at helping with the academic side of building a school, like creating the curriculum.
Maybe Neville could be involved too. He has shown great courage in the original series. In this fanfic, he could be the one who has some prior knowledge about the dragon due to his family's history in the wizarding world. His knowledge could be crucial in helping Harry enter the dragon's territory and deal with whatever lies within.