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 Furies could be his enemies. Since they are guardians of the Underworld and Percy being the son of Charon might disrupt their order. They could try to stop him from using any powers or knowledge he gains from his connection to the Underworld. Another potential enemy could be Hades himself. If Hades feels that Percy is a threat to his rule in the Underworld, he could send his minions after him.
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 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.
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.
Perhaps some minor gods who have been overlooked in the past. They could see Percy's new status as the 'Son of the Universe' as an opportunity to gain more influence and decide to ally with him. For example, the god of small rivers or the goddess of wildflowers.
Annabeth could be a strong ally. She has always been there for Percy, and even though his parentage has changed, her loyalty might still hold. Grover too. He's Percy's best friend and would likely stand by him no matter what. Another potential ally could be the Hunters of Artemis. Since Diana is another name for Artemis, they might see Percy as someone they need to protect and help, especially if he is involved in something that concerns the goddess.
Annabeth could be a great ally. She's smart and resourceful, and she could help Percy figure out the mysteries related to Sobek and his Egyptian connections. Her knowledge of architecture and strategy would be useful in any quests Percy undertakes.
The centaurs from Camp Half - Blood could be allies. They are wise and have a long history of helping demigods. Their knowledge of ancient prophecies and combat techniques could be useful to the Son of Artemis. Additionally, some of the minor gods' children might side with him. For example, the children of Hecate could use their magic to help him in difficult situations. They might see the Son of Artemis as a symbol of change in the demigod world and want to support that. Nico di Angelo, with his powers over the Underworld, could also be an ally. He could offer protection from the darker forces that might target the Son of Artemis.
Annabeth could be one of his allies. Even though Percy is now the 'Son of Eris', their long - standing friendship and her loyalty might make her stand by him. She could help him understand his new situation and use her wisdom to navigate the challenges that come with it.