Well, in this fanfiction, Percy might have all kinds of adventures. He could be tested by the gods in strange ways. Maybe he has to prove his worth again, like facing some new and powerful monsters created by the gods just for him.
Obviously, Percy would be one of the main characters. And of course, the gods would play important roles too. Gods like Zeus, Poseidon, and Athena are likely to be there. Percy's friends from the demigod world, like Annabeth, might also make appearances.
In such a fanfiction, Percy might initially be confused. He's used to Greek gods, and the Celtic gods have their own unique powers and personalities. Maybe he'd have to learn new rules of magic or combat specific to the Celtic realm. For example, the Celtic gods could have different ways of granting boons or curses compared to the Greek ones.
In a fanfiction where Percy Jackson meets the Egyptian gods, it would be a clash of two mythologies. Percy's sword skills and ability to control water might not be as useful as he thinks against the Egyptian gods' unique powers. He could befriend some of the Egyptian gods who share similar values to the Greek gods he knows. However, he might also make enemies among those who see him as an outsider. The fanfic could explore how Percy uses his wits and his knowledge of Greek mythology to navigate the world of Egyptian gods and their associated creatures.
Percy Jackson meeting the Norse gods in fanfiction could lead to some epic quests. The Norse gods might enlist Percy's help in retrieving a lost artifact that has the power to disrupt the balance between the worlds. Percy, with his skills and knowledge from the Greek world, would bring a unique perspective to the task. There could also be some humorous moments as he tries to understand the Norse gods' accents or their rather strange (compared to Greek) ideas of hospitality.
Probably Poseidon, as he is Percy's father and has a vested interest in his son's well - being. Zeus might also be protective in some cases, despite their past differences, as Percy is a hero who often saves the world which benefits Zeus's realm as well.
Zeus is a possibility. As the king of the gods, he might see Percy's potential and adopt him to have an ally among the demigods. Another could be Poseidon, since Percy is already his son in the original story. It would be an interesting twist to have Poseidon formally adopt Percy in a more divine way.
The gods might show their protection in various ways. For example, they could directly intervene in battles Percy is in, using their divine powers to shield him from harm. They might also send him signs or omens to guide him away from danger.
Well, without reading the specific fanfiction, it's hard to say exactly who Percy is. But generally, in a 'champion of the gods' scenario, Percy would be a central figure. He might be a hero with unique powers, like being able to control certain elements or having superhuman strength. He could be a character who has to face various challenges and tasks on behalf of the gods, kind of like a chosen one in a fantasy story.
In a 'Percy Meets Paul' fanfiction, Percy could be a demigod, like in the Percy Jackson series. He has powers related to the Greek gods. Paul, on the other hand, could be a normal human who is very intelligent and curious. Their meeting could be a chance encounter, like Percy accidentally stumbling into Paul's life while on a mission. Or Paul could be someone who has been unknowingly drawn into Percy's world of magic and mystery.