Percy Jackson himself is very likely to be one of the main characters. His life is at the center of much of the action in the original series, so in a fanfiction about lives colliding, he would be a key figure. Annabeth Chase would also be likely involved, as she is Percy's close companion and often involved in his adventures.
Grover Underwood might be involved too. As Percy's best friend, his presence could add an interesting dynamic, especially if his life collides with new characters or situations. Another character could be Nico di Angelo. His unique powers and his somewhat solitary nature could create some really interesting interactions when his life intersects with others in unexpected ways.
Annabeth Chase could also be a main character. She's Percy's close friend and often involved in his adventures, so if lives are colliding in the fanfic, she'd be right there in the middle of it all. Additionally, Grover might be a main character too, as he's Percy's best friend and has his own important role in the Percy Jackson world.
Percy is the main focus, but other demigods could also be important characters. For example, Nico di Angelo, with his powers over the dead, could be crucial in dealing with the 'bringer of the end' situation. Thalia Grace, with her lightning - fast reflexes and leadership skills, might also be involved. And don't forget Chiron, the centaur who has been guiding and training demigods for ages. He could provide valuable wisdom and knowledge in the face of this new threat.
The gods would be involved. Zeus, Poseidon, or any of the other gods. If they make decisions that affect Percy negatively, like sending him on a dangerous mission without proper support, he'd be angry. Also, Clarisse could be in these stories. She and Percy have had their share of conflicts in the original series, and in a fanfic, she could do something to really tick Percy off, like stealing his thunder (figuratively speaking) in a battle or competition.
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.
One interesting plotline could be the characters from different parts of Percy Jackson's world suddenly finding their lives intertwined in unexpected ways. For example, demigods from different camps having to team up against a new, powerful threat that targets all of them regardless of their origin.
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.
Definitely the core Avengers like Iron Man, Captain America, Thor etc. Since it's about the multiverse colliding, we'll probably see their alternate versions too. For instance, a more steampunk - inspired Iron Man from a different universe or a female Captain America.
Percy is, of course, the main focus. However, in many of these fanfictions, Luke Castellan can also be a major character. Since Luke had his own issues with the gods and turned to the dark side in the original series, he could either be an influence on Percy's dark turn or an ally in his new dark journey. Nico di Angelo is another character who could be involved. Given his connection to the Underworld and his own complex relationship with Percy, he might try to understand what's going on with Percy or try to fight against him if he thinks Percy has gone too far.
In this fanfiction, Percy Jackson is the central figure as he controls the Styx. But other important characters could be Grover, his satyr friend. Grover has always been there for Percy and could provide support or comical relief. Also, Nico di Angelo could play a significant role. Given his connection to the underworld, he might have some insights or even a conflict with Percy regarding the control of the Styx.
Well, Percy Jackson himself would obviously be the main character. And since it's about being a slave of the gods, gods like Zeus, Poseidon, and Athena could be important characters as well, as they might be the ones either enslaving him or having some role in his enslavement or liberation.