A group of pure - blood wizards who were against Voldemort but also didn't like the idea of a half - blood hero like Harry could betray him. They might start a smear campaign against Harry, claiming he was unfit to be the face of the wizarding world's new era, despite all he had done during the war.
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.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.
There's a chance that some of his former teammates who have their own insecurities. For example, someone who always felt in Naruto's shadow might be the one to betray him in order to get more recognition for themselves.
Perhaps a new character introduced in the fanfiction. A seemingly friendly newcomer who infiltrates their group, gains their trust, and then betrays them at a crucial moment. This character could be working for an enemy force, either from the Titans or some other ancient power that wants to see the downfall of Percy, Thalia and Nico.
A powerful magician or sorcerer in the fanfiction world might betray them. Such a character could have their own agenda and see Percy, Jason and Nico as obstacles. They could use magic to create illusions and false evidence to make it seem like the three have committed wrongdoings.
It could be Ron. Maybe he starts to feel that Harry has changed too much after the war and is no longer the friend he once knew. He might be influenced by outside forces or his own insecurities. Another possibility is Hermione. If she gets too involved in the new Ministry politics and starts to believe that Harry's actions are not in line with the new wizarding order, she could turn on him.
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.
In some fanfictions, Harry might be ostracized post - war because people are jealous of his fame. He defeated Voldemort and became a hero, but some might resent his status and try to push him aside.
It could be her rival within her group. Maybe they're jealous of Cheetara's skills or popularity, so they betray her to gain an upper hand.
It could be a jealous member of their group. Maybe someone who is envious of Kagome's special powers or the attention Shippo gets. This person might spread false rumors about them, leading to the rest of the group turning against them.
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.