Well, there could be new characters created specifically for this fanfiction. For instance, Chaos might have his own loyal servants or minions who interact with Percy. There could also be other heroes or demigods from different pantheons. They might come to Percy for help or to challenge him. And of course, Percy's mother, Sally. She might be worried about her son and try to reach out to him, even though he is now in the realm of Chaos. Her presence could add an emotional aspect to the story as Percy has to balance his new life with his love for his mother.
Obviously, Percy Jackson himself is a main character. He's the focus as the adopted son of Chaos. Chaos would also be a major character, as Percy's adoptive father. There might be appearances from old friends like Annabeth. She could be worried about Percy in his new situation or even try to help him. Another character could be some of the primordial deities related to Chaos. They might be either allies or antagonists to Percy depending on the story.
It's possible that some of Percy's old enemies, like Kronos' minions who somehow survived, might resurface in this fanfiction. They could be trying to stop Percy from achieving whatever goals Chaos has for him. Also, some of the gods might get involved. They could be curious about Percy's new situation, or they might be worried that Percy's new power under Chaos could be a threat to them. And then there could be some mysterious entities from the Chaos - related realms that we haven't heard of before in the original Percy Jackson stories.
Obviously, Percy would be one of the main characters. Chaos, who raises him, would also be very important. There might also be some other primordial beings that interact with Percy during his upbringing.
Obviously, Percy would be one of the main characters. And then there's Chaos. Depending on the writer, other characters from Percy's world like Annabeth or Grover might also make an appearance, perhaps coming to find Percy in the chaos - filled realm.
The Hunters of Artemis are very likely to show up. They are Artemis's loyal followers and would surely be involved in the search for their mistress. Annabeth could also be there. She's Percy's friend and has great wisdom, which could be useful in devising a plan to save Artemis. Grover might be another character. His ability to sense things could help in tracking Artemis or finding clues about her whereabouts.
Obviously Percy is one of the main characters. There might also be the leaders of the chaos army. They could be some powerful and mysterious figures that Percy has to interact with. And perhaps some of Percy's old friends might also play a role, either trying to bring him back or having to face him as an enemy now.
Maybe in the fanfiction, Percy is on the verge of death in a battle. Chaos, who has been observing Percy for a while, decides to save him and adopts him. Percy then embarks on a journey in Chaos's realm. He could be trained by powerful beings there and form new alliances. His relationships with his old friends from his previous life might be strained as they don't understand his new situation, but he also gains new friends who are loyal to him because of his connection to Chaos.
Besides Percy, his agent could be a character. The agent would be responsible for handling Percy's endorsements, scheduling his competitions, and dealing with the business side of his athletic career. There could also be a physiotherapist, especially if Percy gets injured during his training or competition. This person would help him recover and get back to peak performance. Another possible character is a fan. A die - hard fan of Percy who follows his every move, collects his memorabilia, and is always there to cheer him on, either at the stadium or through social media. And there could be a local sponsor who is investing in Percy and hoping for his success at the Olympics.
In this fanfiction, Percy could be exposed to different realms and dimensions that are part of Chaos' domain. He might meet some very strange and powerful creatures. Maybe he'll form alliances with them or learn from them to become an even greater hero. Also, his personality could change as he adapts to this new and wild environment under Chaos' care.
Maybe it could be due to some powerful magic or a unique prophecy. In many fanfictions, unexpected events like this are often triggered by such elements.