There could be other characters like Sasuke as well. Sasuke's quest for power and his complex personality might bring him into contact with the immortal wanderer. Maybe he sees the wanderer as a threat or a source of knowledge, which would make their interactions an important part of the fanfiction.
Sasuke would be a likely character to appear. Since he and Naruto have a deep connection, Sasuke might cross paths with Naruto during his wanderings, either as an ally or an antagonist depending on the plot.
Since I don't know which 'wanderer novel' you are referring to, I can't say for sure. But it could be a lone wanderer as the main character, someone with a mysterious past.
Obviously, Himawari would be one of the main characters. Since it's about her immortality, she's at the center of the story.
Besides the main trio, characters like Kakashi could still be important. He could be a mentor figure to the immortal Naruto, guiding him through the new challenges that come with his eternal existence. And villains from the original series might also make comebacks to test Naruto's immortality.
The two main characters are the courier and the lone wanderer. The courier might be characterized by his resourcefulness in traversing different regions. The lone wanderer, on the other hand, could be defined by his solitude and a personal mission that he is on. Their personalities and motives will drive the story forward in this fanfiction.
I'm not sure exactly as the details of 'The Wanderer' aren't fully known to me. But typically in a science fiction story, there could be an intrepid explorer as a main character. There might also be a scientist who is there to study the new phenomena they encounter.
The main character is, of course, the dark wanderer. There may also be some side characters like the old wizard who sometimes gives him advice or the young girl who he once saved.
The main character is probably the wanderer. But other possible main characters could be those who are somehow related to the wanderer's past, like a family member or an old acquaintance who he is trying to find or get away from. There could also be characters in the present during his wandering journey, such as a fellow traveler who shares part of the journey with him and has a different perspective on life. This fellow traveler could influence the wanderer's decisions or help him see things in a new light. Additionally, there might be antagonists in the story, perhaps people who try to stop the wanderer from achieving his goals during his travels.
One interesting plotline could be the immortal wanderer's encounters with different villages in the Naruto world. He might share his unique knowledge and skills, which could change the power dynamics in those villages.
Since I don't have the actual fanfic in front of me, it's hard to say exactly. But it could be an original character created by the author who is a lone wanderer in the Naruto world.