His family, if he has any remaining family members at this point, would also be deeply affected. They would be worried sick about his whereabouts and what could have happened to him. And any young witches and wizards who looked up to Harry as a hero would be left confused and disheartened by his sudden disappearance.
He'll most likely meet Bella Swan. Maybe he stumbles upon her in the forest and she's initially very wary of him, but then they start to talk and find they have some similarities in terms of being outsiders in their own worlds. He could also meet the Cullens. Edward might be intrigued by Harry's magic and Harry could be fascinated by Edward's vampire abilities.
One possibility could be that Harry is so traumatized by the war that he wants to isolate himself from the wizarding world. He might go into hiding in a remote, Muggle - free location to deal with his inner demons.
In this fanfiction, Harry may encounter other students who have their own unique stories. Some might be children of Death Eaters trying to make amends by learning about creatures. He could also meet a wizard from a far - off land who has different ideas about how to interact with magical beasts. And there's a good chance he'll meet a werewolf who is trying to control his condition through the knowledge learned at the creature school.
His closest friends like Ron and Hermione would probably be the most affected. They are used to having Harry around and relying on him. His sudden ignoring would leave them confused and worried.
His friends would surely be affected. Hermione and Ron might be both amazed and a bit worried. They could be worried that this new power might change Harry too much or put him in more danger. Also, his enemies like Draco Malfoy would be affected. Draco might become more jealous or even more afraid of Harry.
Uma would be one of the characters affected. Since she was often associated with Harry Hook when he was 'bad', his change might make her reevaluate her own actions and beliefs. She could either decide to follow his lead and also turn towards good or struggle with the change in their relationship dynamic.
Another possible betrayer could be a jealous friend. For example, Neville might feel overshadowed by Harry throughout the war. After the war, this jealousy could turn into resentment, and he might do something to harm Harry's reputation, thus betraying him.
Dumbledore would also be affected. As a mentor - like figure to Harry, he has certain expectations and plans for Harry. If Harry stops caring, Dumbledore's grand schemes against Voldemort might be derailed. He would have to find a new way to motivate Harry or adjust his strategies.
In the fanfiction, Harry could meet Native American wizards or witches who have a deep connection to the land and ancient magic. Their magic might be based on nature and spirits in a way that is very different from what Harry is used to in Britain. He may also meet some Muggle - born American wizards who have a different perspective on being part of the wizarding world, having grown up in a different cultural and social environment compared to Harry.
Voldemort could be the antagonist. Even if he was defeated in the original story, there could be a scenario where some remnant of his dark magic causes Harry and Hermione to disappear. Maybe he had a secret plan that was set to activate after his death and it involved taking them out of the picture.