Main characters Percy will interact with? Well, he'll surely have some sort of encounter with the Fates. Since time travel is involved, the Fates would have a role to play. Maybe he has to convince them not to interfere too much with the changes he's making. Also, he could interact with the Titans in a more personal way. Not just as enemies as in his own time, but perhaps understand their motives better by being in the past with them. And don't forget about the minor gods. In the past, they might have had more influence or different relationships, and Percy could get caught up in all of that.
Obviously, Percy and Kronos are the main ones. Percy, the demigod son of Poseidon, is likely to be the hero of the story. Kronos, the powerful Titan, is the antagonist that Percy has to face.
One possible plot twist could be that Percy meets his own parents at a different time than expected. For example, instead of seeing his mother as he knew her, he might encounter her as a young girl. Another twist could be that Kronos has some ulterior motive for sending Percy back that Percy doesn't initially realize, like changing the course of events to actually benefit Kronos in the long run. Maybe Percy discovers that by changing certain events in the past, he's unknowingly setting up a different future that Kronos can manipulate more easily.
Obviously, Percy Jackson and Kronos are the main characters. Percy is the hero, the son of Poseidon, with his powers over water and his ability to lead. Kronos is the powerful Titan, the main antagonist, with his devious plans to take over the world.
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.
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.
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.
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.
Naruto would be a very likely character to interact with Hinata. Since they have a strong connection in the original story, in a time - travel scenario, their relationship could develop in new and interesting ways. Maybe Hinata could be more open with her feelings earlier in time.
Maybe there are some original characters created by the fanfiction author. These characters could have their own unique abilities and backstories. They might be the ones who tip the scales in Percy's favor, for instance, by having a special power that weakens Kronos just before Percy makes the final blow.
Percy is bound to be central. Alongside him, some new characters could be introduced. Maybe there are demigods who are against Percy's return because they see him as a threat to their own status. But the main gods would still be major players. They could be portrayed in different lights, some being more desperate to get Percy back than others, which could lead to interesting power struggles among them.