Maybe there's a prophecy at camp that some misinterpret to mean Percy will bring destruction. So, the campers, out of fear for their safety, betray Percy. They could lock him up or try to send him away, even though Percy has always been loyal to the camp. He then has to find a way to either clear his name or find a new place where he belongs.
In some fanfictions, a new camper could be secretly an enemy spy. This new camper befriends Percy and gains his trust. Then, the spy uses Percy's trust to access important areas of the camp and steal valuable items. When the theft is discovered, Percy is wrongly accused of being in on it, and the camp betrays him. Percy has to fight to expose the real culprit.
In Camp fanfiction, Percy could be betrayed when the camp leaders are deceived by false evidence against him. Let's say an enemy creates some fake proof that Percy has been colluding with their side. The leaders, without fully investigating, might turn on Percy. Also, there could be a love - related betrayal. If Percy has a romantic interest in the camp, the person he likes could be forced by their family or some external pressure to break Percy's trust and use his feelings against him, for example, to make him give up his position in the camp or some important secrets.
One possible plot could be Percy being framed for a crime he didn't commit by the Olympians. Maybe they needed a scapegoat for something that went wrong in the mortal world that they didn't want to take the blame for. Another plot might involve Percy discovering a dark secret about the Olympians that they don't want him to reveal, so they turn on him to keep him silent. It could also be that a new powerful enemy has influenced the Olympians against Percy, perhaps by spreading false rumors or using some form of mind control on them.
One possible plot could be that Tails is betrayed by a close friend who becomes jealous of his intelligence and skills. For example, Sonic might start to feel overshadowed by Tails and in a moment of weakness, turn against him. This could lead to Tails having to fend for himself and prove his worth all over again.
One possible plot could be Percy being betrayed by someone he trusted in the context of Artemis' world. Maybe a fellow demigod turns on him due to jealousy of his relationship with Artemis. Another could be Artemis herself being betrayed by a member of her hunt, and Percy gets caught in the middle trying to help her.
One possible plot could be that Boruto is betrayed by his friends due to a misunderstanding. For example, they might think he has turned to the dark side because of some false evidence planted by an enemy. Another could be that Boruto is betrayed by the village he loves. Maybe the village leaders suspect him of a crime he didn't commit and turn against him.
The betrayal of Percy by Camp Half - Blood in fanfiction can also be a way to show Percy's growth and resilience. He has to face the ones he considered family turning against him. This can lead to some really interesting character development arcs, where Percy has to re - evaluate his loyalties and find new allies or even learn to rely more on his own strength.
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.
One possible plot could be that Fleur betrays Harry in a Triwizard Tournament - related task, perhaps by secretly helping the other champions. Another might be that Harry discovers Fleur has been passing information about the Order of the Phoenix to the wrong side. Maybe Fleur betrays Harry's trust regarding a personal secret, like his feelings for Ginny, and uses it to her advantage in some way.
Perhaps in the fanfiction, Natsu is betrayed by someone he trusts in a mission. For example, they might leave him behind in a dangerous situation to fend for himself while they take the credit for the mission. Or it could be that a love interest betrays Natsu by choosing someone else over him due to some false perception or external pressure.
One possible plot could be that Naruto's family, perhaps his father or mother, were actually alive but had to fake their deaths for some dark reason. Then they reappear and turn against Naruto, maybe because they are being controlled by an evil force. Another plot might be that his family had some long - hidden secrets that made them see Naruto as a threat to their real goals, so they betrayed him.