There could be new characters as well, like a devil mentor figure who teaches Percy about his devil heritage. This character could be ancient and have a lot of knowledge about the devil realm. Or there could be an angelic character who is initially an enemy but later realizes Percy is not fully evil and they end up forming an unlikely alliance.
He could interact with his old friends at Goode, like Rachel. She has a special connection to the demigod world, and their relationship could be further explored. Also, there might be new teachers or students that Percy meets. Maybe a new student who is actually a demigod in hiding too, and they start to figure out each other's secrets.
In addition to Annabeth and his family, he might interact with other demigods he knew before. They could be in awe of his new godlike state. There could also be some characters from the godly realm who are interested in Percy's return. They might want to use his power for their own purposes or they could be there to guide him on how to fully control his newfound abilities.
There could be a character who is a mentor to Percy in understanding his devilish side. This character might be an ancient being with knowledge of both the divine and the demonic. For example, a fallen angel who has seen the rise and fall of many with such mixed heritages.
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.
Obviously, Percy would be the main character. There could also be some characters from the original Percy - related universe who might make an appearance to help him adjust or just to check on him. And from the Forks side, characters like Bella or Edward (if it's in the Twilight - Forks setting) could potentially cross paths with Percy. Maybe they become friends or foes depending on how the story unfolds.
Maybe there is a social worker named Mrs. Johnson who is trying to help the homeless in the area. She tries to reach out to Percy, but he is initially hesitant because he has trust issues. These characters can all interact with Percy and shape his story while he lives on the streets.
Well, aside from Percy himself, there could be a wise old owl as a mentor figure. The owl could teach Percy about the secrets of the wild, like which plants are safe to eat and which are poisonous. There might also be a wild horse that Percy befriends and rides. This horse could be his loyal companion during his adventures in the wild.
Obviously Percy is the main character. There could also be other characters from the original series, like Annabeth. She might be worried about Percy's transformation and try to help him adjust. And maybe some of the gods would also be involved, either trying to control Percy or use his new powers for their own ends.
Obviously Percy himself. There could also be some of his friends from his normal life who come to visit him in the wild or maybe even join him, like Annabeth. And then there could be various wild animals that act as characters, like a wise old owl or a friendly deer.