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.
It could be one of Percy's own half - siblings. Jealousy might play a role. If Percy is getting too much attention or is seen as the favorite of Poseidon, a half - sibling could spread false tales about him, causing others to believe he is not trustworthy and thus betray him.
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.
Luke could be a likely candidate in some fanfictions. He already had his own issues and motives in the original story, and it wouldn't be a stretch for a fanfiction writer to take that further and have him betray Percy in a more elaborate way.
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.
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.
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.
Well, Ares could be involved. He has had his clashes with Percy in the original stories. In fanfiction, his war - like nature and desire for chaos could make him set up Percy to look like he's betrayed the community. Also, Octavian from the Roman side might be a character in such a scenario. He's power - hungry and jealous of Percy's status. He could spread false rumors or create situations that make it seem as if Percy has betrayed the Roman demigod community.
His mother, Sally Jackson, could play an important role as an ally. She has a strong will and has always been there for Percy. Also, Tyson, his cyclops half - brother, could be a powerful ally with his strength and loyalty. And don't forget about the Nymphs. Some of the water nymphs might help Percy since he has a connection to water.
Percy Jackson himself would obviously be a main character since it's a 'Reborn Percy Jackson Fanfiction'. His friends like Annabeth Chase are also likely to be main characters. She could play a crucial role in helping Percy adjust to his new life after being reborn. Grover would probably be there too, still being Percy's loyal friend. And the gods, especially those who have a connection to Percy like Poseidon, would also be important characters as they might have different reactions to Percy's rebirth.
Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.