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.
Jiraiya could be a part of the fanfiction. He might have known something about Naruto's sister all along but kept it a secret. He could then train her in some special ninja techniques. Rock Lee could also be involved. His enthusiasm for training and becoming a great ninja could inspire Naruto's sister. And Neji could be there as well, with his Byakugan skills perhaps being used to protect Naruto's sister from hidden threats.
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 possibility could be an original character created specifically for the fanfiction. This gives the writer the freedom to shape her personality and backstory completely. Another could be a minor character from the Naruto series that doesn't get much attention, like Tenten's sister if she had one. It could also be a reimagined version of Hinata as Naruto's sister. Since Hinata has her own story in the original series, reimagining her as Naruto's sister with a neglected past could lead to some interesting fanfiction ideas.
Marinette is often a popular character. Since she has a relationship with Adrien in the original series, in the fanfiction, she could interact with the sister in various ways. Maybe they become friends, or there could be some initial jealousy from Marinette if she thinks the sister is getting too much of Adrien's attention.
Obviously, Naruto and his sister are the main characters. Naruto is a well - known character from the Naruto series, known for his determination and his dream of becoming Hokage. His sister, though not part of the original series in this way, would be a new character in this fanfiction with her own unique traits and abilities.
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.
Well, obviously Naruto himself would be a main character. His sister could be an original character created just for the fanfiction. In terms of the harem members, Sakura could be one. Even though she has feelings for Sasuke in the original series, in this fanfic she could see Naruto in a new light. Hinata is also a likely candidate as she already has deep feelings for Naruto. Another could be Ino, who might be drawn to Naruto's growth and newfound confidence in this alternate scenario.
Sasuke could be involved. Since he has a complex relationship with Naruto, if Naruto becomes 'Red X', Sasuke might be either an ally or an antagonist depending on the nature of 'Red X'. Another character could be Kakashi. As Naruto's mentor, he would surely notice the change and try to guide Naruto through it.
Ron could also play an important role. He could be a bit jealous at first of the close bond between Harry and his sister, but then they become friends. Ron's sense of humor and loyalty would make him a fun addition. He could be involved in their adventures, like when they go looking for Horcruxes, and provide some comical relief during tense times.
Sakura could be involved. She could be like a healer or an advisor in the new world. Her medical skills would be very useful. Also, Sasuke might be there. He could be a sort of enforcer, using his Sharingan to detect threats. In the world of Asgore, they could form a new kind of team, with Naruto as the leader like Asgore. And then there could be characters from Asgore's world like Toriel. She could be a guide to Naruto about the culture and history of the place.
Sasuke could be involved. He might be one of the ones making fun of Naruto, in his typical aloof way. Or he could be in the background, just observing and secretly finding it a bit funny while pretending not to care. Sakura could also be there. She might scold the others for making fun of Naruto, or she could join in the teasing a little bit out of her own insecurities.