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.
In some fanfictions, Percy's half - brother Tyson might be made to betray him. This could be due to being tricked by some sea monster that convinces Tyson that Percy is a danger to their kind. Also, Grover could be lured into a situation where he has to choose between saving the wild and betraying Percy. It often makes for a very heart - wrenching story as Grover and Percy have such a strong bond. Sometimes, the betrayal comes from an unexpected source like the gods themselves. They might use Percy as a pawn and then turn on him when he's no longer useful.
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.
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.
One common theme is the shattering of trust. When Percy gets betrayed, it's all about how that trust he had in someone is completely broken. This often leads to him feeling isolated and alone.
Percy would likely feel a great sense of hurt and confusion. He might question his own judgment in trusting Annabeth. He could become more withdrawn and less trusting of others in general.
The gods, especially Zeus. He has always been a bit suspicious of Percy. In fanfiction, he could easily be written as spreading lies about Percy to the other gods and demigods, leading to everyone turning against him. Also, some new characters introduced in fanfiction might be the ones who start the chain of events that lead to Percy's all - around betrayal.
One possible reason for Annabeth to betray Percy in fanfiction could be a misunderstanding. Maybe she was misled by false information about Percy's actions or intentions. For example, if someone spread rumors that Percy was secretly working against the demigods, Annabeth might believe it without getting the full story.
Another aspect could be that Percy is betrayed by a family member. For example, if it is revealed that a relative he trusted was working for an enemy all along. This would shatter Percy's world as family has always been important to him, and it would be a major plot point in the fanfiction, leading to his emotional and physical struggles.
He may also try to figure out what went wrong. Percy is a smart demigod, and he would likely not just accept the betrayal without trying to understand the cause. So, he might start investigating, talking to other characters who might have some insight, like the other demigods or even some minor gods who are friendly to him.