His mother, Sally Jackson, could play an important role as an ally. She has a strong will and has always been there for Percy. Also, Tyson, his cyclops half - brother, could be a powerful ally with his strength and loyalty. And don't forget about the Nymphs. Some of the water nymphs might help Percy since he has a connection to water.
Annabeth is a very likely ally. Their relationship is so strong that she would follow him into Tartarus and be by his side through thick and thin. Grover could also be an ally. He's Percy's long - time friend and his skills as a satyr could come in handy in Tartarus. Nico di Angelo is another possibility. His knowledge of the Underworld and his powers over the dead could be useful in the dangerous Tartarus environment.
Draco Malfoy would be a likely character. Given his Slytherin status, he would likely have a complex relationship with Percy. He might start off as hostile, seeing Percy as an outsider, but then they could end up having to work together for some reason.
One character that could betray Percy is Thalia. She has her own issues with the gods and her place in the world. Maybe she gets jealous of Percy's growing popularity and power among the demigods. So, she decides to side with an enemy to take Percy down a notch. Additionally, some minor characters that Percy has helped in the past could be the ones to betray him. They might feel indebted to him but also resentful at the same time. An enemy could manipulate their feelings and turn them into traitors against Percy.
Ares could be a likely character. He's always been somewhat of a wild card in the Percy Jackson series. He might betray Percy for his own amusement or to gain more influence in Olympus. Another could be Zeus. He's often paranoid about his power and might see Percy as a threat and betray him to protect his throne.
One likely opponent is Luke Castellan. Since they have a complex relationship in the original series, it would be a very emotional and intense battle in fanfiction. Luke has his own set of skills and powers, and his motives would make the battle even more interesting.
I think it would be Magnus Chase. Since Magnus has experience with the Norse world and Percy is used to dealing with the demigod stuff, they could combine their knowledge and skills. Also, they both have a sense of justice and would likely work well together.
Athena might also be a good candidate. Although she and Poseidon have their differences, she could see Percy's potential and befriend him. She could offer him wisdom and strategic advice for his various adventures.
Percy is the main character so he'll be there. Annabeth will probably be by his side as she often is in his adventures. Leo Valdez could also be a character in this fanfiction. His mechanical skills might come in handy if there are any traps or devices related to the Host of Apophis. Piper McLean could be involved too, using her charm - speak to influence certain situations. And Thalia Grace, the daughter of Zeus, could make an appearance. Her leadership skills and her ability to call lightning could be important in the fight against the Host of Apophis.
Well, Clarisse might be paired with a more aggressive Doom character. She's a fierce warrior in the Percy Jackson series. They could form a powerful duo, fighting side by side against the common enemies. Also, Tyson could be paired with a strong, hulking character from Doom. Tyson is a Cyclops with great strength, and finding a counterpart in Doom that shares that physical power would create an interesting dynamic in the fanfiction.