Perhaps Luna Lovegood. She has such an open and imaginative mind that she'd be perfect for exploring the mysteries of space. She might see things that others can't and offer unique insights. And Dobby the house - elf could also stow away on their spaceship. He'd be eager to help Harry in this new adventure, using his magic to keep things tidy and maybe even find some hidden magical treasures in space.
Ron might go with him too. He's always been by Harry's side through thick and thin. They could have some hilarious moments in space, like trying to eat space - themed wizarding food that Hermione made for them. And Neville could also join them. He's grown to be a very capable wizard, and his knowledge of plants might come in handy if they encounter any strange space flora.
Hermione would be a great candidate. She's smart and could research all about space magic before they go.
Most likely Hermione. She is always looking out for Harry's well - being and would want to make sure he gets the flu shot properly. She would also be curious about the Muggle medical procedure.
Well, he might use magic to create a breathable atmosphere around him. Maybe he'd encounter strange alien creatures and use spells like 'Stupefy' to defend himself.
It could be an older wizard who has turned his back on the traditional ways that the group in the fanfic adheres to. Maybe he has discovered a new form of magic that is considered taboo by the others, and so they see him as an outsider, not one of them. He might also be trying to bring about changes in the wizarding world that the group is not ready to accept, such as more integration with non - magical people in the fanfic's story.
Hermione could accompany him. She's always been by his side and would likely follow him out of loyalty and a thirst for new knowledge. Together, they could explore new magical theories in different parts of the world. Another possibility is Hagrid. He has a soft spot for Harry and might want to protect him on his journey. He could introduce Harry to different magical creatures in other countries.
It could be one of his friends like Ron or Hermione. They often accompany Harry and would be likely to be near him.
Dumbledore could also play a role even if he's no longer alive in the normal sense. His portrait in the headmaster's office might give Harry some cryptic advice about the sword. Or perhaps his spirit could appear to Harry in a moment of need and guide him on how to use the sword effectively in the fight against the forces of evil.
There could be new villains like an evil wizard - scientist who wants to use magic to create black holes and destroy planets for his own gain. Or perhaps an alien race that has a natural ability to drain magic from wizards and witches and they see the Hogwarts students as a source of power.
Ron and Hermione are the most likely ones. They are his best friends and have always been by his side through thick and thin. They would probably follow him without hesitation and use their own skills, like Hermione's vast knowledge and Ron's loyalty and practical skills, to assist in the ship - stealing adventure.
The Malfoys might oppose him. They are always looking for ways to undermine Harry, and if his inheritance gives him more power or status, they would surely be envious and try to stop him.
There could be some new characters introduced in the fanfic. Maybe a powerful wizard from a far - off land who has heard of Harry's plight and decides to help. Or perhaps a group of outcasts in the wizarding world who see Harry as a symbol of their own struggle against the unjust system and come to his aid.