Luke Castellan could also be a character in these fanfictions. Even though he had his own story arc in the original series, in fanfiction, he could be brought back in some way related to Percy's revenge. Maybe Percy discovers that there was more to Luke's actions and he has to get revenge on someone who was behind Luke's fall to the dark side, and Luke's spirit or memories could be involved in the process.
Some of the gods can also be common characters. For instance, Poseidon, Percy's father. He might play a role in either helping or hindering Percy's revenge, depending on the plot. Zeus could also be involved if Percy's revenge somehow crosses paths with the affairs of the sky god. And then there are the other demigods at Camp Half - Blood who could join Percy in his quest for revenge or oppose him if they have different views.
One interesting plotline could be Percy seeking revenge on a new enemy who has wronged his friends at Camp Half - Blood. Maybe this enemy has been stealing magical artifacts from the camp, and Percy decides to use his water powers and his wits to track them down and make them pay.
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.
The other Hogwarts professors like McGonagall and Flitwick might be included. If Snape is getting revenge on students who misbehaved towards him, characters like Draco Malfoy or the Weasley twins could be part of the story. Draco might be involved if he had previously mocked Snape, and the Weasley twins could be targets if they had played pranks on Snape that he now wants to pay back.
Perhaps it's a gift from his father, Poseidon. Since Poseidon has control over the sea and all things related to water, he might have some connection to a power that gives Percy wings, like as a way to help Percy reach new heights both literally and metaphorically.
There might be new gods or minor deities introduced. They could be ones that have been forgotten or ones that are from a different part of the mythological world. Their appearance could change the power dynamics in the story. They might either help or hinder Percy in his second - chance journey.
Well, of course, Percy Jackson himself often makes an appearance. He could be a mentor figure to the adopted son, guiding him through the ropes of being a demigod. Annabeth is also common, perhaps helping the adopted son with her wisdom and knowledge of the mythological world.
Percy Jackson himself is, of course, a common character. Annabeth Chase often appears as well, as she has a significant relationship with Percy in the original series and is likely to be part of the harem in these fanfictions. Grover might also show up as Percy's friend, providing some comical relief and support in the story.
Percy Jackson himself is very common, as the main character of the original series. Writers often like to put him in new and chaotic situations.
Chiron is also likely to be a common character. He would be the one to help Percy adjust to his new identity as a son of Demeter. And there could be antagonists too. Maybe some monsters that are specifically targeting Demeter's children because of their connection to the earth and its resources. They could be trying to disrupt the balance that Demeter's children help maintain.
Common characters include Percy Jackson, of course. Then there's often Annabeth as she is Percy's main love interest in the original series. Some fanfictions might also feature Clarisse La Rue, adding some conflict to the harem dynamic as she and Percy have had their differences in the past. Gods and goddesses like Aphrodite might also make an appearance, perhaps using her powers of love and beauty to either enhance or disrupt the hypnosis harem situation, given her domain and nature.