A group of bandits might be the main antagonists. They see Percy as someone they can easily rob or enslave since he is all alone. Percy would then have to use his skills to outwit them.
Perhaps an ancient evil force that has been awakened due to the circumstances of Percy growing up alone. This evil force could be attracted to Percy's loneliness and the potential power he may develop as a result, and it wants to corrupt him for its own purposes.
It could be a jealous demigod who sees Percy's wings as an unfair advantage. This character might try to find a way to cut off Percy's wings or steal the power that gave him the wings.
It could be a new character. Maybe a dark sorcerer who has come to Neverland while Peter was growing up. This sorcerer wants to take over Neverland and sees Peter, with his new adult understanding and power, as the only obstacle. The sorcerer could start to corrupt the island, turning the mermaids and Lost Boys against Peter, making him fight not only for his place in Neverland but also for the very soul of the island.
Well, one possibility is that Percy has to learn to fend for himself in a harsh world. Maybe he discovers unique powers within himself as he faces various challenges alone. For example, he could learn to hunt and build shelters all by himself, much like a survival story.
It could be Tartarus itself personified. As Percy is alone in this terrifying place, Tartarus could take on a form and directly oppose him. It would use all the horrors and challenges of the place to try and break Percy.
It could be Nyx. She is a very powerful primordial goddess in Tartarus. Since Percy is alone, she might see him as an easy target or a threat to her domain. Nyx could send all sorts of dark creatures and illusions to stop Percy from achieving whatever goal he has in Tartarus.
It could be a new character introduced specifically for this fanfiction. Maybe a bully who has just transferred to Percy's school and takes an instant dislike to him. This bully might be jealous of Percy's popularity or has a grudge against him for no real reason other than wanting to assert dominance.
It could be a new dark wizard who targets Harry because of his fame. This wizard might be jealous of Harry's status as the hero who defeated Voldemort and wants to prove himself as more powerful.
Well, for this type of fanfiction, the plot might start with Percy being separated from his usual allies very early on. He'd be forced to navigate the mortal world by himself. Since he's growing up alone, he may not have the same understanding of his Greek heritage as he does in the original series. This could lead to him making different choices when it comes to using his powers. For example, instead of using them to protect Camp Half - Blood, he might use them to simply survive in a world that seems to be constantly against him.
Well, Percy is obviously one of the main characters. Since it's about him growing up on Olympus. Annabeth might also be a major character as she has a close relationship with Percy. She could be visiting him on Olympus or they could be having some long - distance communication that affects the story.
Well, Percy is the central figure for sure. There might be a hunter named Thalia who could be a main character. Thalia has her own backstory and her presence could greatly influence Percy's experience in the Hunt. Additionally, some new characters created specifically for this fanfiction could also be main characters, ones that interact with Percy on a regular basis and help shape his journey as he grows up with the Hunt.