One possible reason could be jealousy. Maybe his sister is jealous of Percy's special abilities or the attention he gets. Another reason might be that she is forced to abandon him due to some external threat or danger that she believes will harm her if she stays with Percy. Or perhaps she has been brainwashed into thinking that Percy is a bad influence or a danger to her.
Well, sometimes in fanfiction, the story might set up a situation where Percy's sister abandons him to protect him in a very convoluted way. She may think that by leaving him, he will be forced to become stronger on his own and be able to face some greater threat in the future. Or it could be that she has made a deal with some sort of dark force in order to save someone else in the family, and part of the deal is to abandon Percy.
In some fanfictions, it could be related to a family feud. There might be some long - standing issues within the family that cause his sister to abandon Percy. For example, if there are different loyalties within the family, and his sister sides with those who oppose Percy. It could also be that she has her own self - centered goals and sees Percy as an obstacle to achieving them, so she abandons him.
It could be that there was a prophecy related to his sister that made the family think she was more important. For example, a prophecy that she would be the key to saving the village in the future. And in their short - sighted view, they abandoned Naruto to focus all their resources on her.
Percy might be initially very confused and hurt. He may try to find his sister and understand why she has left him. He could also become very angry and feel betrayed, which might lead him to take some rash actions.
One reason could be jealousy. If Naruto is getting more attention or has achieved more in the ninja world, his sister might be jealous and this could lead to her actions that hurt Naruto, either by spreading false rumors or directly confronting him in a hurtful way.
It could be that his sister has some special abilities or talents that the family or the village values more. For example, if she has a very rare and powerful bloodline limit, everyone's attention would be drawn to her. In a ninja world, power and potential are highly regarded, so Naruto might be overlooked because his sister seems to have more of that 'special something' at first glance.
One possible plotline could be that Naruto's family is in a difficult situation and they believe they can only support his sister. So, Naruto is left on his own. He then has to struggle to survive in the ninja world without the support of his family. He might become more independent and self - reliant, forging his own path.
Well, sometimes in fanfiction, the author wants to explore the theme of loneliness. So they create a scenario where Naruto's friends abandon him because they are influenced by the village's elders. The elders might spread false rumors about Naruto to maintain their control. His friends, being young and naive, believe these rumors and turn their backs on Naruto, which then leads to Naruto's journey of self - discovery and growth in solitude.
In some fanfictions, it could be due to his parents being on a dangerous mission. They might have been forced to leave Naruto in order to protect him from the enemies who were after them. Maybe they thought that by abandoning him, the enemies wouldn't be able to find him as easily. Another reason could be that they were under some sort of dark magic or curse that made them act against their will. This could lead to them leaving Naruto without any choice.
It could be because Percy is different. If he comes from a different background or has a unique personality trait, the bullies might target him. In the fanfiction, if Percy is from a different race or has an unusual way of speaking, the bullies might pick on him for not being like them.
Love could be the main reason. Since he loves Annabeth deeply, he doesn't want to lose her, so he becomes possessive.
One reason could be for a fresh take on the story. Writers might want to break away from the typical demigod narrative and explore a darker, more supernatural side of the Percy Jackson universe. Another reason could be to create a more complex and morally ambiguous character. By making him a demon, they can introduce new conflicts and relationships.