As twins, they would have a special connection. They could start to have a sort of unspoken understanding. For example, when one of them is in danger, the other could sense it. Their relationship would be a mix of brotherly love, friendly competition, and mutual respect. They would grow together, both in their magical abilities and in their understanding of the wizarding world and their place in it.
A possible plotline is their different magical abilities. Percy might be really good at transfiguration and charms, while Harry has his natural talent for defense against the dark arts. They could compete in a friendly way, but also learn from each other. For instance, Percy could teach Harry some advanced transfiguration spells, and Harry could show Percy some new defense techniques he learned from his experiences.
They would likely raise them with a strong sense of honor and courage. Percy would teach them about the power of the sea and how to use it for good. Annabeth would instill in them a love for knowledge and strategy. They would also train their twins in combat skills to protect themselves in the dangerous world of demigods.
In Percy Jackson, the gods themselves would have mixed reactions. Zeus might see the twins as a threat to the balance of power between the worlds. Poseidon and Athena, on the other hand, might be more intrigued and want to use the twins to their advantage. In the Harry Potter world, the students at Hogwarts would be divided. Some, like Ron, might be a bit wary, while others, like Luna, would be very open - minded. The Death Eaters would likely see the twins as something to be exploited if they thought it could help their dark cause.
Well, they could be the result of some sort of cross - over experiment gone wrong (but in a good fanfic way). Maybe they have powers from both worlds but struggle to control them at first. For example, they might accidentally cast spells while trying to use their demigod powers. And they could be the bridge between the two groups of friends. The demigods are skeptical of wizards and vice versa, but the twins help them understand each other.
One idea could be that the twins have powers from both the wizarding world and the demigod world. They can use magic spells like in Harry Potter and also have the abilities of a demigod from Percy Jackson. For example, they can control water like Percy and at the same time cast defensive spells. Another plot could be that they are sent on a mission to stop a threat that endangers both the wizarding world and the Camp Half - Blood. They have to use their combined knowledge and skills to succeed.
For the Harry Potter characters, Ron would probably be a bit jealous of the twins' cool powers at first. But as he gets to know them, he would become friends. Snape would be highly suspicious and might even try to find a way to control them. In the Percy Jackson realm, the other demigods would have mixed reactions. Some would be in awe of their magic, while others would be worried that they might bring too much attention from the wizarding world. The twins would have to work hard to gain the trust of both sets of characters.
If Percy and Thalia were twins, their relationship with Luke would change significantly. Luke might see them as a bigger threat to his plans. He might try to drive a wedge between them, knowing how strong they could be together. Their relationship with Chiron would also be different. Chiron would have to train them in a way that takes advantage of their twin connection, perhaps teaching them special combat moves that only work when they work in tandem. The gods would also treat them differently. They might be more watchful of the twins, fearing their combined power could disrupt the balance among the demigods.
In many fanfictions, the twins' mischievous side is explored further. They might come up with even wilder pranks than in the original books, showing their creativity and love for chaos. For instance, they could turn the Gryffindor common room into a giant maze for a day just for fun.
In the fanfiction, perhaps Harry has a special ability or spell that helps him tell the twins apart. Or it could be as simple as the twins deliberately giving him some secret signs or signals to help him differentiate. Another possibility is that Harry notices differences in their magical auras. Each person's magical aura can be unique, and Harry, being observant, might have detected this difference between the twins.