Maybe she's jealous of Naruto's popularity or his strength. It's possible that she feels overshadowed by him all the time and that jealousy turns into resentment, leading her to hurt Naruto.
Another way is if his sister has some special powers that she can't fully control yet in the fanfiction world. And during a training session or an accidental encounter, her powers lash out and end up hurting Naruto.
Well, Naruto could also seek support from his friends like Sasuke or Sakura. They have been through a lot together, and they might be able to offer him some comfort and advice on how to handle this difficult situation. Since Naruto values his friendships highly, their words could have a big impact on him.
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.
In some fanfictions, Percy's sister might hurt him emotionally by betraying his trust. For example, she could share his secrets with others, making him feel exposed and betrayed.
In the Naruto series, Naruto doesn't have a biological sister in the original story. So in this fanfiction, the author probably creates a new character to be his sister. It could be an original character with unique abilities and a complex backstory that leads to the situation where Naruto dies by her hand.
A common scenario could be over inheritance or family heirlooms. If there's something valuable in the family that both Percy and his sister want, she might scheme to get it all for herself, leaving Percy feeling cheated and hurt.
She could be an original character created specifically for this fanfiction. The author might give her unique abilities and a distinct personality to make the story more interesting.
One possible reason could be to protect her. Naruto might believe that by leaving, he can draw danger away from his sister. For example, if there are powerful enemies targeting him, he doesn't want his sister to get caught in the crossfire.
The relationship could start with Naruto being overly protective of his sister, but she wants more independence. This causes friction. Later, when she gets into trouble that only Naruto can help her with, he steps in. This makes her realize that his over - protectiveness comes from love, and they begin to repair their relationship. For instance, if she is kidnapped by a powerful enemy, Naruto uses all his skills to rescue her, and during this process, they have heart - to - heart conversations.