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.
There might also be an antagonist, perhaps another alien who sees Harry as a threat for some reason. This character could be trying to stop Harry from achieving something, like learning the full extent of his powers or finding his place in the alien world.
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.
I don't know for sure as it depends on the specific fanfic. There could be a boss - like character who oversees Harry's work in the unit.
There could be a mentor figure, perhaps an older and more experienced ice elemental who guides Harry in controlling his powers. Another character might be a friend from a non - elemental background who is trying to understand Harry's new identity.
Maybe there's a character named Jack, who is a bit older than Harry and has been on the streets for a long time. He acts as a sort of mentor to Harry, showing him the ropes. And then there could be a young mother named Sarah who is also living on the streets with her baby. Harry might help her out sometimes, and she in turn offers him a sense of family.
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.
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.
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.