Another likely betrayer could be a god or goddess who feels that Percy has become too powerful or has disobeyed them in some way. They might use their influence to turn the campers against Percy and have him banished, all as part of their own power play in the world of demigods.
It could be a new character introduced into the fanfiction. Maybe someone from a different camp who has a hidden agenda. They pretend to befriend Percy but then betray him to further their own goals, which could involve taking over Camp Half - Blood or getting access to a powerful weapon.
A possible betrayer could be a jealous demigod. There are always those in the Percy Jackson world who envy Percy's abilities and the attention he gets. This demigod might see an opportunity to get rid of Percy and make themselves look better in the eyes of the other campers.
It could be a fellow student who was secretly jealous of Harry all along. Maybe someone like Draco Malfoy. He had always been at odds with Harry and in a fanfiction, he could be pushed to the point of betraying Harry for his own self - centered reasons. Another possibility is a teacher. A teacher who was influenced by the dark side or had some hidden agenda could betray Harry. For example, if there was a teacher who was secretly working for Voldemort or a new dark wizard.
It could be some of the pure - blood supremacists who were biding their time. Even though the war was over, they might still resent Harry for his role in defeating Voldemort and decide to get revenge by making it look like he betrayed them. Another possibility is some of the Ministry officials who were corrupt and didn't want Harry's influence around.
Another possibility is that it could be an old acquaintance who has changed for the worse. They might have been jealous of Ash and Lyra's success or popularity in the Pokémon world. This person could use their knowledge of Ash and Lyra's habits and weaknesses to betray them. It could also be a group of people rather than just an individual, like a team of thieves or poachers who see Ash and Lyra as obstacles to their illegal activities.
A jealous rival could be a likely betrayer. Maybe someone who is envious of Bell's popularity or his relationship with certain characters. For example, another adventurer who feels overshadowed by Bell might betray him to gain an advantage. Another possibility is a character who is being manipulated by a darker force. They might not have wanted to betray Bell originally but are forced to under the influence of some evil power.
Another type of betrayer could be a family member. In some fanfictions, family dynamics can be complex. A family member might betray fem Percy out of a sense of duty to a different cause or because they believe she is a threat to the family's reputation or safety.
It could be Ares. He is often portrayed as a violent and unpredictable god. Percy might banish him if he goes too far in causing wars and chaos among mortals. His banishment would have a huge impact on the battlefields of the mortal world.
Gary Oak could be a likely betrayer. He has always been Ash's rival, and in some fanfiction, he might take things too far and betray Ash in a crucial Pokémon battle, like using underhanded tactics to win. This would surely make Ash cry.
One character that could betray Percy is Thalia. She has her own issues with the gods and her place in the world. Maybe she gets jealous of Percy's growing popularity and power among the demigods. So, she decides to side with an enemy to take Percy down a notch. Additionally, some minor characters that Percy has helped in the past could be the ones to betray him. They might feel indebted to him but also resentful at the same time. An enemy could manipulate their feelings and turn them into traitors against Percy.
Ares could be a likely character. He's always been somewhat of a wild card in the Percy Jackson series. He might betray Percy for his own amusement or to gain more influence in Olympus. Another could be Zeus. He's often paranoid about his power and might see Percy as a threat and betray him to protect his throne.
Apollo might be the betrayed god. He could have been tricked by another god into betraying Percy. For example, Apollo might have been promised more power or a higher status among the gods if he turned against Percy. But then he realizes too late that he's been used and that he's actually betrayed someone who could have been an ally.