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.
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.
The main characters would be Carlisle and Harry. Carlisle is the adoptive father, so he'll be responsible for Harry's well - being in this new family. Harry, of course, is the central figure as the adoptee. However, we can't forget about the rest of the Cullen clan. Alice, for example, might use her power to foresee any potential dangers that could come to Harry. Jasper could help Harry with self - control, as he has his own struggles with it. So, all these characters would be integral parts of the fanfiction.
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.
Hades himself would be a central character. His influence on Mal, his parenting style, and his own motives for raising Mal would be important aspects of the story.
The main characters are obviously Percy and Nico. Percy, a well - known and powerful demigod, and Nico, who has his own unique powers and a rather troubled past.
It could be Percy and Black Widow. Natasha is a master of reading people, and she'd notice Percy right away. She might approach him in a more stealthy way compared to the others, trying to assess if he's a threat or an ally. And Percy, being used to danger, would be on his guard at first, but then start to trust her as she shows her good intentions.
The main characters are Percy and Fem Hades. Percy is the protagonist from the original series, and in the fanfiction, his story continues with his interactions with Fem Hades. Fem Hades, on the other hand, is a unique take on the Hades character. She could have different personality traits compared to the male Hades in the original mythology. There might be other characters that are introduced as the story progresses, but these two are the foundation of most Percy Jackson and Fem Hades fanfiction.
Obviously, Naruto and Inoichi are the main characters. Naruto is the one being adopted, and Inoichi is the one doing the adopting.
Obviously, Harry would be the main character. He's the one with the unique heritage of being Hades' son. Then there would probably be Hades himself, who could play a big role in guiding or sometimes hindering Harry. Hermione and Ron would likely still be important characters as Harry's friends, though their relationship with Harry might be more complex due to his new identity. And there could be other demigods or underworldly beings like Persephone, who might have some influence on Harry's journey.
Obviously, the oc reborn as Issei would interact with Rias Gremory. She is a central character in the High School DxD world and as Issei is part of her peerage, the oc in Issei's place would have a lot of interaction with her. This could range from her guiding the oc in the ways of the devils to them having some sort of romantic or at least a very close friendship relationship.