You can start by giving her a distinct personality. If Percy is brave and impulsive, make her more cautious and calculating. For example, she could be the one who always thinks twice before jumping into a fight, analyzing the situation and coming up with a plan. This contrast in personalities can create interesting dynamics within the story.
She could be developed as more carefree compared to Percy. Percy is often uptight and by - the - book. His twin sister could be the one who's always breaking the rules in a harmless way, like sneaking out to the Forbidden Forest just to explore. This would make her a more rebellious character, which is a big contrast to Percy.
Another plot could involve the twin sister being cursed. This curse could be related to her connection with the sea as well. For example, every time she uses her powers related to the sea, she experiences a great deal of pain. Percy then has to find a way to break the curse. In the process, they discover an ancient prophecy that not only involves Percy but also depends on his sister's role in fulfilling it.
It's possible that they have a complementary relationship. If Percy is still the more studious and rule - following type, his twin might be the more adventurous and creative one. They would then balance each other out, with Percy keeping his twin in line when needed and the twin getting Percy to step out of his comfort zone.
Maybe they'd develop some sort of combined spell - casting technique. Since Percy is good at academic magic and Harry has his own natural talent, they could create a unique form of defensive magic.
It would be a huge shift. Since Harry is the Boy Who Lived, the family would have to deal with even more attention from the wizarding world. Percy, being Harry's twin, might feel overshadowed or might try to use Harry's fame to boost his own career ambitions. And the younger Weasleys would have a different kind of role model in Harry compared to what they had with just Percy.
One plot idea could be that Percy's twin sister is the opposite of him in terms of Hogwarts house. While Percy is in Gryffindor, she could be in Slytherin, causing a lot of family drama. Another idea is that she has a special magical talent that Percy is jealous of, like being able to communicate with magical creatures in a unique way. Or perhaps she gets involved in a secret society at Hogwarts that Percy disapproves of, leading to conflicts between them.
In fanfiction, Percy being a twin could make him more competitive. Since he has a twin, he might constantly strive to outdo his sibling, which is different from his character in the books where he mainly focuses on his own academic and career pursuits.
The twin brother's character might be developed based on his past. If he had a difficult childhood, it could make him more reserved or cautious. But as he gets to know Naruto and Sakura, he starts to open up. He could start as a loner type, but as he gets involved in their adventures, he becomes more social and confident. He might also develop his character by exploring his own unique abilities and how they can be used for good.
Princess Yaya's Gemini Spirit Ring was a ring inlaid with two blue stars. It was given to Yaya by the previous king of the Gemini constellation before he passed away. This ring was a Gemini constellation artifact and had a special connection with the Gemini King. Yaya could use the energy of the twin spirit rings. She could use this ring to impart magic to others. Under certain circumstances, the twin spirit rings might be hidden and one would need to use their senses to find them.
While waiting for the TV series, you can also click on the link below to read the original work of " Little Fox Demon Matchmaker " to understand the plot in advance!