Dumbledore might play a role too. Even in death, Dumbledore could be guiding Harry as a ghost child. He might offer wisdom and advice on how to handle his new state, or he could be the one with a plan to reunite Harry with the living, if that's what is meant to be in the story.
Ron could also be a main character. He may be initially scared of Harry's ghostly presence but then tries to help him. He might be the one who goes on adventures to find a way to reverse Harry's situation, like looking for a rare magical object or an ancient spell.
Well, Dumbledore would most likely be a significant character. He might be one of the few people who quickly understand the implications of Harry being a mutant. He could help Harry navigate through the challenges that come with his new identity. Additionally, there could be some mutant characters from the mutant universe who cross over into the Harry Potter world. For example, a character like Professor X could be an interesting addition, guiding Harry in understanding and using his mutant powers in a responsible way.
Maybe there's a human friend who is really curious about Harry's new situation. This friend could be constantly asking questions and trying to get involved in Harry's alien - related adventures, adding some comedic relief to the story.
Hermione would definitely be one of the main characters. She would likely be very concerned about Harry and try to find a way to reverse his mental regression. She might spend hours in the library researching spells and potions to cure him.
Another character could be a fellow inmate who has been in jail for a long time. He befriends Harry and teaches him the ropes of surviving in jail.
It could be Hermione. Since she's Harry's close friend, she would likely play a significant role in a story where Harry is immortal. She might be the one trying to help Harry adjust to his new state or be involved in his adventures in some way.
Well, Gandalf would probably be a main character too. He is a powerful wizard in Middle - earth and his knowledge and guidance would be crucial in a story where Harry is involved. He could be the one to help Harry understand the differences between his magic and the magic of Middle - earth. Also, Aragorn could be part of it. His leadership and warrior skills could be something that Harry looks up to or tries to learn from in this fanfic.
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.
Another character could be a shy first - year boy from a different house, say Hufflepuff. He might be too timid to speak up in class, and Harry helps him gain confidence. His name could be Oliver. Harry's help could be crucial in making Oliver feel more at home in the magical world.
Perhaps Sirius Black could also be involved. Sirius was Harry's godfather and was very much a part of Harry's life. His presence could add some interesting dynamics, like he could be the one who stumbles upon the magic that makes the meeting possible and then has to deal with the implications of Harry meeting his mother.
A group of misfit soldiers could be main characters too. They are all a bit different and don't quite fit the typical military mold. But together with Harry, they form a unique unit. There might also be a high - ranking officer who takes an interest in Harry's progress and either helps or hinders him depending on the situation.