Perhaps it's someone who has been influenced by an enemy. For example, a demigod who has been tricked or bribed by a powerful foe of Percy's. This person might not have been a bad person originally but due to the outside influence, they end up betraying Percy.
It could be a jealous friend. Maybe another demigod who is envious of Percy's powers or his popularity among their peers. This friend might see an opportunity to get Percy out of the way and so betrays him.
It could be a new acquaintance who Percy trusted too quickly. This person could have been pretending to be Percy's friend all along but was actually working for an opposing force. Once they got the chance, they betrayed Percy's trust and caused him to seek refuge in the Hunt.
There's a chance that it could be a character from Percy's family. For example, a distant relative who has been corrupted by some dark magic. They might betray Percy for their own gain or because they are being controlled, and this betrayal could lead to Percy being killed.
Well, perhaps Percy is betrayed by his friends or allies who he thought he could trust. This betrayal could be due to jealousy, a misunderstanding, or someone's hidden agenda. As a result, Percy feels so hurt and angry that he decides to join the Hunt. In the Hunt, he might find a new sense of purpose, new friends who understand his pain, and he could start to train hard to become a more powerful fighter. He may also seek revenge against those who betrayed him.
The camp leaders might betray them. If there is a power struggle within the camp, and Percy and Nico are seen as threats to the current leadership's power, the leaders could set them up. They could make it look like Percy and Nico are breaking camp rules or endangering the other campers, which would lead to the rest of the camp turning against them.
Maybe a character from their past who has a hidden grudge. Say, someone they defeated in a previous battle who has now found a way to get revenge. This person could pretend to be their ally again and then stab them in the back when they least expect it.
It could be a jealous camper. Maybe someone like Clarisse, who has always had a bit of a rivalry with Percy. She might see an opportunity to get rid of him and make herself look better in the eyes of the camp. So, she could plant false evidence or spread lies that lead to Percy's betrayal.
The gods are always a possibility. Zeus, for example, has been distrustful of Percy in the past. If Zeus feels that Percy is getting too powerful or is a threat to his rule, he could engineer a situation where Percy is betrayed. He might use other demigods or even mortals to turn against Percy.
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.
Obviously, Percy is one of the main characters. There might also be the leaders of the hunt, whoever they are in the fanfiction's setting. Maybe there's an experienced hunter who acts as a mentor to Percy during the hunt.
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.