Percy himself, of course. Given his leadership skills and powerful abilities, he would be the first to step up and face the Pillar Men. His ability to control water could be a great asset or a hindrance depending on the situation.
Well, Annabeth would definitely be one. She's always been by Percy's side, and in this fanfiction, she might be a bit worried about Percy being the champion of Nyx. She could be trying to help him while also being cautious of Nyx's motives.
Draco Malfoy would be a likely character. Given his Slytherin status, he would likely have a complex relationship with Percy. He might start off as hostile, seeing Percy as an outsider, but then they could end up having to work together for some reason.
Grover might also be involved. His ability to sense emotions and his connection to nature could play a role. He could be the one to initially sense Padme's arrival or help her understand the more magical and nature - related aspects of the Percy Jackson world, like the power of the gods and the spirits of the forest.
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.
Probably other demigods would interact with Percy. Maybe some demigods who are in need of a more stable influence in their lives would seek out Percy. Also, characters like Grover might find Percy in Hestia's care and there would be a reunion. And the gods and goddesses who are associated with the concept of home or family in Greek mythology, like Demeter, could also have interactions with Percy.
Definitely Annabeth. She and Percy have a strong relationship, and Hera's possessiveness would surely disrupt that. Also, other demigods at Camp Half - Blood could interact with Percy. They might sense that something is off with Percy due to Hera's actions and try to help him or at least figure out what's going on. And of course, the gods themselves might interact with Percy. Some gods might side with Hera, while others might oppose her possessiveness over a demigod like Percy.
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.
Obviously, Percy and Hades will interact a great deal. Percy will also likely interact with the Furies, as they are servants of Hades. They could be like his protectors or trainers in some sense. Another character could be Nico di Angelo, since he has a connection to Hades as well. They could bond over their shared relation to Hades.
The characters could interact in various ways. Percy could use his ability to understand Ancient Greek to communicate with the cavemen if they have some sort of connection to the old language. For example, if they are descendants of an ancient Greek - influenced civilization. Or, Percy might use his powers to perform feats that the cavemen would see as magic, and they would either revere or fear him based on that.
In these fanfictions, the demigods often try to understand the SCPs from their own perspective of good and evil. They might see some SCPs as new kinds of monsters to defeat. On the other hand, some SCPs could be seen as potential allies if they share a common goal. For instance, if there is an SCP that is being hunted by a more dangerous entity, the demigods could form a temporary alliance with it. The interactions are full of tension and discovery as the two different worlds collide.