It could be a rogue ninja who has a grudge against Naruto. This ninja might have developed a special jutsu that can manipulate memories. He uses it on Naruto's friends to make them forget Naruto, hoping to isolate and break Naruto's spirit.
Maybe an old enemy of the Hidden Leaf Village. They see Naruto as a symbol of the village's strength. By making his friends forget him, they aim to weaken the village. This enemy could be someone from a long - forgotten clan who has been planning revenge for years and finally found a way to target Naruto through his friends' memories.
He would likely feel extreme loneliness. Naruto has always valued his friends, so being forgotten by them would be like losing a part of himself. He might start to doubt his self - worth, thinking that if his friends could forget him so easily, then maybe he wasn't as important as he thought.
One possibility is that Naruto might have made a great sacrifice that his friends didn't fully understand at first. Maybe he had to go on a dangerous mission alone and was presumed dead. His friends, in their grief and shock, gradually started to forget about him as they moved on with their lives. For example, they could have been so focused on protecting the village from new threats that they didn't have time to dwell on Naruto's absence.
The plot might involve a curse. Naruto could be cursed by an ancient power. As a result, his friends' memories of him start to fade away. He might notice this when they stop responding to him as they used to. Naruto would be heartbroken but determined. He could search for the origin of the curse, facing many challenges along the way. And in the process, he might discover some long - lost secrets about his own family or the ninja world that are related to this curse.
It could be a jealous relative who wants Naruto out of the picture so they can have more attention or power within the family.
The main character is surely the forgotten son. But we can't rule out other important characters. Sakura could be in the story, maybe she discovers some clues about the forgotten son. And Sasuke might also be involved, as he has a long - standing relationship with Naruto. Their children could also interact with the forgotten son, adding more layers to the story. Additionally, there could be some new characters created specifically for this fanfiction, like a wise old hermit who guides the forgotten son on his path.
Since I don't know the specific fanfiction, it's hard to say. But it's likely to include the forgotten doctor, and perhaps some Naruto characters like Naruto himself, Sasuke or Sakura who might interact with the doctor in some way.
It could be a demigod who was once shunned and now wants revenge. This demigod has turned to the dark side and uses their powers in a twisted way to cause chaos among the other demigods. They might target the forgotten demigod specifically because they see them as a symbol of all that is wrong with how demigods are treated.
It could be Mysterio. He's always been jealous of Peter's connection to Tony. He uses his illusions to mess with Tony's mind and make him forget Peter. Mysterio wants to be the one in the spotlight and sees Peter as an obstacle, so by making Tony forget him, he can get rid of Peter's influence on Iron Man.
Naruto is most likely one of the main characters. After all, it's a Naruto Shippuden - related fanfiction.
Obviously Naruto himself would be a main character as the story is about him being forgotten by his family. His parents, if they are alive in this fanfiction, could also be important characters. They are the ones who are forgetting him, so their actions and reasons for this forgetfulness would be crucial to the story. And perhaps his friends like Sasuke and Sakura could also play important roles. They might be the ones to support Naruto when his family fails to do so, and they could also help him in his quest to figure out what's wrong with his family.