You can start by giving them unique personalities. For example, one could be really shy but have a hidden strength. Then, put them in different situations to show how they grow. Like when they face a monster, the shy one might find the courage to fight.
Think about their backstories. If they are adopted, what was their life before? Maybe one was an orphan on the streets and had to be really tough to survive. This would influence how they interact with others in the fanfiction. Also, give them relationships with the existing characters. They could have a mentor - mentee relationship with Percy or Annabeth.
To develop the characters, explore their fears and desires. Let's say one of the adopted children desires to be accepted by the other demigods. Their fear could be of being rejected because they are different. As the story progresses, they could overcome this fear through their actions and the support of their new family. You can also show their development through their dialogue. If at first they are hesitant to speak up, later they become more confident and start leading conversations.
One idea could be that the adopted children have special powers related to different elements, like one can control water in a unique way different from Percy. They could go on a quest to find a lost artifact that is key to their new family's safety.
Another character that could be included is Clarisse. Her tough - as - nails attitude could contrast with the niece's, and they could start off as rivals but eventually become friends. Also, Tyson, Percy's cyclops half - brother. He could be like a big, protective figure for the niece, and his unique abilities could come in handy during adventures or when the niece is in danger.
In fanfiction, Percy and Hermione's characters can be developed in various ways. Some writers might explore Percy's sense of duty and how it interacts with Hermione's thirst for knowledge. For example, they could be on a mission together where Percy's strategic planning, which he learned from his time in the Ministry, combines with Hermione's research skills to solve a complex magical problem.
Perhaps Percy's niece is suddenly transported to a different realm in the world of Greek mythology. This realm is in chaos due to some dark magic. She has to use her wits and any latent abilities she has to survive and find a way back. Meanwhile, Percy is on a quest of his own but starts to notice strange things that lead him to suspect his niece is in trouble. Eventually, their paths cross and they team up to save the day in this strange new realm.
One idea could be that Percy's niece discovers she has a unique power that no one else in the family has. Maybe she can communicate with sea creatures in a different way than Percy. Another could be that she gets involved in a quest that was thought to be only for the main heroes in the Percy Jackson world, and she has to prove herself to the other demigods.
A rogue magician who has a vendetta against Percy's family. He sees Percy's niece as an easy target to get back at Percy. He uses his magic to manipulate the world around her, causing confusion and danger. He might also be trying to steal her powers for himself to gain more influence in the world of magic and mythology.
Obviously, Percy is one of the main characters since the title implies he's central to the story. The other main characters would likely be the leaders or prominent members of the Hunt. They would play important roles in Percy's adoption process and his subsequent life within the Hunt.
For Percy, you could explore his insecurities more deeply. Maybe he's worried about living up to his reputation as a hero. With Emma, give her unique traits. For example, she could be really good at deciphering ancient languages. Then, put them in situations where they have to rely on each other. Like when they're trapped in an ancient maze.
Well, of course, Percy Jackson himself often makes an appearance. He could be a mentor figure to the adopted son, guiding him through the ropes of being a demigod. Annabeth is also common, perhaps helping the adopted son with her wisdom and knowledge of the mythological world.
For Dark Percy, his character could be developed by showing the internal struggle between his dark side and his original self. Flashbacks to his normal self can be used to highlight the contrast. As for Zoe, her development can be centered around her growing trust in Dark Percy despite his changed state. She could start as a cautious observer and gradually become a fierce defender of him.