Perhaps a centaur who was once a great teacher but was forgotten after a tragedy at the camp. He could have been teaching some ancient and powerful magic that got lost with him. The centaur might have been betrayed by someone and thus became forgotten, but now his knowledge is needed again.
It could be an old demigod from a long - forgotten camp. Maybe someone who was exiled or lost during a great war in the past. For example, a demigod who had the power to control the weather but was shunned because people feared their power.
It could be a demigod who was overshadowed by the more well - known ones like Percy. Maybe someone who was born to a minor god or goddess and got lost in the shuffle. For example, a demigod of a nature god who was always in the background while the big battles were fought by the children of the more prominent Olympians.
In Percy Jackson father fanfiction, the sea god Triton could be an antagonist. He might be jealous of Percy getting so much attention from Poseidon. The primordial sea gods or goddesses could also be antagonists. They might see Poseidon's relationship with a half - blood as a perversion of the natural order. And then there are always the minor gods who might feel that Percy and Poseidon are getting too much power and try to undermine them.
Frank Zhang could also be an ally. He is a Roman demigod with shape - shifting abilities. His strength and loyalty would make him a valuable teammate for Percy. Additionally, Nico di Angelo, who has ties to both the Greek and Roman worlds, could be an important ally. His knowledge of the Underworld and his ability to shadow - travel could help Percy in various situations, whether it's getting information or quickly moving around in the Roman camp.
Annabeth would definitely be one of his top allies. She's been with Percy through many adventures and her intelligence would be crucial in helping Percy understand his connection to the creator and any new situations that arise. For example, she could research ancient texts to find clues about the creator's intentions for Percy.
Nico di Angelo could be a great ally. Since he has his own connection to the Underworld, he might understand Percy's new situation better. Another could be Thalia, as she's always been a strong and loyal friend. She would likely stand by Percy despite his new dark powers. Also, Grover could be an ally. His empathy and loyalty would make him support Percy no matter what.
Annabeth could be one. She's been Percy's close friend and has a strong connection with him. Also, Piper could be a part as she has her own charm and abilities. And maybe Thalia, with her fierce nature.
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.
How about a daughter of Hecate? She could have magical powers that would complement Percy's skills. The Fates might bring her into Percy's life because they know that her powers could be crucial in future battles or quests. And then there's Calypso. She already has a connection with Percy, and the Fates could make it so that she becomes a more permanent part of his life, adding to his 'harem' situation.
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.