The Giants. They are known for their strength and hatred towards the gods. If Percy is now a Chaos Champion, they might view him as a major obstacle in their goal of overthrowing the Olympians. They could team up with other dark forces, like the Furies who are also always eager to cause chaos among the demigods.
The Titans could be potential enemies. Even though they've been defeated before, some of them might be resentful and see Percy as the Chaos Champion as a new threat to their possible plans of a comeback. Kronos, for example, might have some of his essence lingering and try to possess other powerful beings to go against Percy.
The children of Ares might see Percy as a threat. Since Cupid represents love and Ares represents war, their natures are quite opposite. The Ares kids could be jealous of Percy's powers or simply not trust him because of his different lineage.
The traditional enemies of the demigods, like the Titans or the Giants, could still be his enemies. They might see him as an abomination because of his father being Charon. Also, some of the more puritanical demigods who follow the Olympian gods strictly might oppose him.
The Titans could still be a threat. Even though they were mostly defeated, there could be some remnants or new Titans emerging. For example, a Titan who was in hiding and now sees Percy as a threat to his plans of taking over again.
There could be new characters introduced as allies. Maybe a demigod from a far - off camp who has special powers related to the sky, just like Zeus. This demigod could understand Percy's new role on a deeper level and they could team up to face the challenges together. Or perhaps an ancient spirit that was once bound by Zeus but now sees Percy as a way to freedom and in return offers its power and knowledge to Percy.
The main character is Percy Jackson for sure. However, depending on the creativity of the fanfiction writer, there could be a whole host of other characters. We might see some ancient beings from the very origins of chaos. There could be other demigods from different cabins at Camp Half - Blood that we haven't seen much of in the original series but play important roles here. And don't forget about the villains. There could be new antagonists who are trying to either take advantage of the chaos or stop Percy from fulfilling his role as the champion of chaos. Annabeth would most likely be there to provide her intelligence and strategic thinking to Percy's more impulsive nature.
I'm not sure specifically as there can be many different fanfictions with this title. But generally, it might involve Percy Jackson in some new adventures related to chaos. Maybe he has to face new threats or make alliances in a world where chaos is a major factor.
Perhaps some of the minor gods who are tired of the traditional power structure in Olympus could be Percy's allies. They might see his rebirth as a son of Chaos as an opportunity to shake things up. Also, any being that has a connection to the concept of change or new beginnings could potentially side with Percy. This includes spirits of innovation or even some forgotten heroes who sense that Percy is the key to a new era in the world of demigods and gods.
It might be a mortal who has been given special powers by the gods for a specific purpose. This mortal could be an unlikely hero, but through a series of events, they are chosen to be the 'Champion of the Sun'. They would then have to learn to control their new - found sun - related abilities and face the challenges that come with the title.
There could be a chaos spirit named Eris. Eris could be a mischievous character who both helps and hinders Percy on his journey with chaos. She might have the ability to shape - shift into different forms related to chaos, like a swirling vortex of dark energy.
Obviously Percy Jackson himself could be one of the characters. Annabeth Chase would be a likely partner as they have a long - standing relationship in the Percy Jackson series. Another possibility could be Nico di Angelo. He has a complex character arc and could be involved in a make - out session in a story that explores his relationships in a more romantic context during a chaotic situation.