It's also possible that she is a character from the Naruto universe who has been given a new role as Naruto's sister in this fanfiction. For example, Sakura could be reimagined as Naruto's sister in a really creative and different take on the story, and then Naruto has to beat her in a battle of wits or strength.
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.
Perhaps in this fanfiction, his sister has done something wrong or dangerous, and Naruto has to 'beat' her in the sense of stopping her. It could involve a moral or ethical conflict within the story that Naruto has to resolve by overpowering his sister's actions.
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.
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.
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.
It could be an original character created just for the fanfiction. Many fanfiction writers like to introduce new characters to add their own unique twists to the story.
Sasuke could be involved. He might be initially suspicious of Naruto's new - found sister but later come to respect her skills. Kakashi could also play a role as a mentor figure to both Naruto and his sister, guiding them in their ninja training. Another character could be Hinata. She could befriend Naruto's sister and help her adjust to life in the village.
She could be very sad. After all, Naruto is likely an important person in her life. She might feel abandoned and start to question why Naruto left her.
She could be a really strong - willed character. Since she's from the world of Naruto, she might have a fierce determination to protect her village and those she loves. Maybe she's also curious about Harry's world of magic and wants to learn more, showing her inquisitive side.
It could be a member of Naruto's family who is fueling the neglect. Maybe an older relative who believes that the sister has more potential or is more worthy of attention and resources. This person could actively try to keep Naruto down.
The sister could be bullying Naruto out of envy. If Naruto has some special abilities or talents that she doesn't, she might be jealous and lash out at him.