Another possibility is that an existing side character from the Naruto series is given the immortality aspect and becomes a main character in this fanfic. For example, a character like Kakashi. If he were immortal, it would completely change his story arc and his relationships with other characters, making him a central figure in the 'immortal thred naruto fanfic'.
It could also be that an original character created for this fanfic is the main focus. This character, being immortal, would have their own backstory and motives that are central to the plot. They might befriend or oppose Naruto and his friends, leading to various adventures and conflicts.
Well, one interesting concept could be the idea of immortality within the Naruto universe. It might explore how a character with immortality would interact with the existing power dynamics and the ninja world's values. Maybe it shows how they deal with the passage of time while others around them age and die.
Typically, in a fanfic that combines Naruto and Bleach elements and involves immortality, you would expect to see Naruto Uzumaki. He is a central character in Naruto and his story could take a whole new turn with immortality. Also, Ichigo Kurosaki from Bleach would probably be there. His unique powers and his journey could be greatly affected by the concept of immortality. There might also be other important characters from both series, like Sasuke from Naruto or Rukia from Bleach, who play important roles in the new narrative of the fanfic.
I'm not sure specifically as I haven't read the fanfic. But usually in a story about love, there might be two main protagonists who are in love with each other. They could be from different backgrounds, like one might be a supernatural being and the other a normal human, which would create interesting dynamics.
Typically, Ruby, Weiss, Blake, and Yang are still main characters in 'immortal rwby fanfic'. Their characteristics might be enhanced or changed due to the immortality factor. For instance, Ruby's leadership skills could be further developed over time. Weiss may use her long life to master more complex magic. Blake could have more time to fight for equality in different historical periods, and Yang's strength and determination might reach new levels as she has all the time in the world to train.
If it follows the typical pattern of a story, aside from the immortal dragon, there might be a human or elf character who is either an ally or an enemy of the dragon. There could also be other magical creatures that play important roles in the fanfic, like a wise old wizard or a powerful demon that the dragon has to face or work with.
Batman is obviously one of the main characters. His presence is central as the story is likely based around his immortality. Robin could also be a main character, as he is often Batman's sidekick and their relationship would be different with Batman being immortal.
Obviously, Himawari would be one of the main characters. Since it's about her immortality, she's at the center of the story.
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.
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.
I don't know the main characters in this fanfic as I haven't read it. But in MLP, common main characters could be Twilight Sparkle, Pinkie Pie, Rainbow Dash, etc. Maybe this fanfic has a new or less - known character as the main focus along with some of the regular ones.