Well, it could make his breathing underwater effortless. He can explore the underwater world more freely without worrying about running out of air. It might also give him an advantage in battles underwater, as he can stay submerged for as long as he needs.
Another possibility is that it was a side - effect of a powerful curse or blessing. For example, if Percy was cursed to be more connected to the water, the gills could be a physical manifestation of that. Or if he received a great blessing from the ocean itself, the gills could be part of that package, making him more in tune with the underwater realm.
One idea could be Percy discovers he has a new ability related to his gills that allows him to communicate with underwater creatures in a more profound way. Maybe he can understand their ancient languages and has to use this new skill to solve an underwater mystery that threatens both the sea world and the surface world.
You can try fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. AO3 has a wide range of Percy Jackson fanfics, and you can use the search function to specifically look for ones related to 'gills'. FanFiction.net also has a large collection, but the quality can vary more. Another option is to check out Percy Jackson - related forums where fans might recommend their favorite 'gills' fanfics.
Maybe it's a strange genetic experiment gone wrong. Some evil scientist in the fanfic world might have tampered with Naruto's DNA, adding some sort of aquatic - like genes that resulted in gills.
Well, having gills would obviously make him better at underwater activities. He could swim faster and stay underwater for longer periods, which could be useful in battles near water or in underwater missions.
πThe novel I recommend to you is Ballad of the Dividing Sea. It's a fantasy romance novel about a different continent. In the plot of the novel, the fragile ecosystem of the water world collapsed, causing the underwater world to become hell. The protagonist listened to the call of fate and looked back at himself. He became a mermaid who could live underwater after being modified and explored many mysterious underwater continents. I hope you like this fairy's recommendation. Muah ~π
With his other demigod comrades, it could change the dynamics in group situations. Some might look at him with pity, which Percy would hate. Others might be inspired by his courage in the face of cancer. It could also lead to some new alliances forming as they all try to find a cure or a way to help Percy manage his condition.
Well, let's say Percy has a disease that affects his powers. His relationship with his demigod friends could change. They might start to see him as a liability instead of a strong leader. And his relationship with the gods could also be altered. The gods might either try to use his weakened state for their own ends or completely abandon him, which would have a huge impact on how he views them and his place in the world.
Well, in some fanfictions, Percy might use his water - controlling abilities. Since poison can be in liquid form, he could manipulate the water within the poison, perhaps separating out the harmful components. Another way could be through his demigod powers, which might give him an innate resistance or control over harmful substances like poison.
His scar might make some people more intimidated by him. They could see it as a sign of his dangerous experiences and be a bit afraid to get too close. For example, new acquaintances in the fanfic might be hesitant to approach him at first.