His sense of responsibility would grow exponentially. Since he's the only one left, he feels it's his duty to preserve the magic world. He could start to develop new magical abilities as he has all the resources and time to experiment. He might also struggle with his identity, as so much of who he was was tied to his relationships with others. Now that they're gone, he has to redefine himself and what it means to be Harry Potter in this new, lonely world.
Harry might become more introspective. He has all this time alone to think about his past, his family, and the things he's been through. He could also become a bit paranoid, always on the lookout for threats since he has no one to back him up. He might start seeing the magic world in a different light, now that he's the only one there to experience it.
His character would likely become more self - reliant. Without his friends and mentors around, he'd have to depend solely on himself. For example, he'd need to make all his own decisions without Hermione's advice or Ron's support.
One possible plotline could be that Harry Potter wakes up alone and at first he's in shock. He might then start exploring the empty world. He could visit Hogwarts which is now deserted. He might find himself reminiscing about his friends and the good times they had. He would have to figure out how to survive on his own, like growing his own food in the Hogwarts gardens which were once tended by the house - elves. He may also stumble upon some magic that was hidden when the world was full of people, and this new discovery could either give him hope of bringing others back or make him more resigned to his lonely existence.
Harry's character would be more attuned to the deeper magics. He would be less likely to jump into things without thinking. Since Flamel is a master of alchemy, Harry might be more interested in the scientific aspects of magic. He would also have a greater respect for life and magic's power, as Flamel, with his long life, would instill in Harry a sense of wonder and caution about the forces they deal with.
Harry's character would transform significantly. Goku's carefree and positive attitude would rub off on him. He would be less brooding and more open - minded. His magic training might take a backseat initially as he gets immersed in learning martial arts from Goku. But as he grows, he would find a way to balance both. He would develop a new sense of self - worth, knowing that he is part of Goku's family. His relationships with others would also change. He might be more assertive in his friendships, and his leadership skills would improve as he watches Goku lead in battles. His magical abilities would become more fluid, as he adapts to using them in tandem with his newfound physical abilities.
He'd be more disciplined. His character would be shaped by military values. He'd be less likely to act impulsively and more likely to think things through strategically.
He'd be tougher. Hiei's no - nonsense attitude would rub off on him. Harry would be more likely to take risks without hesitation.
It would show Harry as a more intellectual and curious character. His love for math could lead him to explore areas of magic that others haven't. For instance, he might use math to predict the movements of magical creatures, making him seem more of a wizard - scientist.
He would become more adaptable. Middle - earth is so different from the wizarding world, so he'd have to learn new ways. His character might become more in tune with nature like the elves in Middle - earth.
If Harry wakes up with Fleur, it could be a bit of a shock for both of them. But then, they might find that they are drawn to each other's personalities. Fleur's elegance and confidence might attract Harry, and Harry's courage and kindness could win Fleur over. Their relationship could develop as they face challenges together. For example, they might have to fight off some dark wizards who are after a powerful magical artifact. Through these experiences, their bond could strengthen into a deep and loving relationship.
His character might become more sophisticated. The French culture's influence could make him more refined in his manners.
His character would become more rugged. Instead of the somewhat sheltered and naive Harry we see at the start of the original series, he'd be tough - minded. Burt Gummer's influence would make him more of a fighter from the get - go.