Naruto is the main character for sure. Hinata might also be in the fanfic. If Naruto is left to die, Hinata could be one of the few who tries to go against the decision and look for him. There could also be some of the older generation characters like Jiraiya. Even though he might be dead in the original story, in the fanfic he could appear in Naruto's memories or as a spiritual guide to help Naruto when he is in such a desperate situation.
Obviously, Naruto would be one of the main characters. Depending on the specific plot, Sasuke could also be involved. He might be one of the ones who either helps Naruto or is part of the reason Naruto is left to die. Sakura could also play a role, maybe she has a change of heart towards Naruto in the fanfic.
The two main characters are Naruto and Rias. Naruto, with his ninja skills and his journey of growth in his original story. Rias, whose actions towards Naruto are the central focus of this fanfiction. There could be side characters like Naruto's friends from his world who might try to find out what happened to him, or characters related to Rias who could either support or oppose her decision to leave Naruto to die.
Perhaps some of the other ninja villages see Naruto as a potential danger in the future, so they manage to infiltrate Konoha and make it seem like Naruto has committed a great crime, resulting in him being left in the forest. In this case, the main antagonist would be the ninja from the other villages.
I'm not sure specifically as there could be many different 'naruto is left to die' fanfics with various plots. But generally, it might involve Naruto being in a situation where his comrades abandon him or he is placed in a deadly situation by some malicious force, and then he has to struggle to survive on his own.
Danzo could be the main villain. He is known for his underhanded tactics and his dislike for Naruto. He might see Naruto as a threat to his power and plan to get rid of him by leaving him in the Forest of Death. Danzo could have manipulated some other characters to make it seem like it was a legitimate mission gone wrong.
Percy is surely one of the main characters, as the story is centered around him being left for dead. There might also be the people who left him, though we don't know who they are yet. Maybe there will be a character who discovers Percy and helps him, like a kind - hearted stranger.
In such a fanfic, perhaps Naruto is betrayed by his village and left in the forest. Maybe there are enemies lurking in the forest that he has to face alone. It could also involve Naruto's struggle to survive and his journey to find out why he was abandoned like this.
Since it's a Naruto - based fanfiction, Naruto is definitely a main character. In addition, it could feature the Hokages, as they often play important roles in Naruto's life and they might be involved in the mystery of the Shinigami's left hand. Also, villains from the Naruto series could show up. For example, Orochimaru might be interested in this power of the Shinigami's left hand and try to take it from Naruto, making him a major character in this fanfiction as well.
Maybe a mysterious ninja from a hidden clan shows up. This ninja is intrigued by Naruto's situation of being left behind by Sakura and decides to help him. They could have some really cool powers like the ability to manipulate shadows in a different way than the Nara clan. Also, there could be a healer from a neighboring village who comes to Konoha. She helps Naruto deal with his emotional wounds from being left behind, and in the process, they become good friends.
I think Minato could also be a main character. As Naruto's father, he might play a role in Naruto's life even though Kushina is absent. He could be a guiding figure or a source of mystery for Naruto, especially if Naruto discovers things about his father in relation to his mother's absence.
Obviously, Naruto is the main character. He is the one who becomes this 'god - like' figure in the fanfic. But there are also other important characters like Sasuke. Sasuke might play a role in either supporting Naruto's new power or being in conflict with him because of it.