It's likely that there is a female protagonist who is on a quest related to the ring. She might be strong - willed and intelligent. And then there could be a male character who is initially her rival but later becomes her ally in the journey involving the ring. There might also be a sidekick character, like a small magical creature that helps them along the way.
There's no way to be certain without reading it, but perhaps there's a female lead who is somehow connected to the ring. Maybe she has a special ability that is awakened by the ring, and there could be a male character who helps her on her journey with the ring. There could also be an antagonist who wants to steal the ring for their own malicious purposes.
Obviously Naruto would be one of the main characters. There might also be his friends like Sasuke and Sakura. But since it's a fanfiction involving a special millennium ring, there could be new characters created specifically for this story who are also important.
Some fanfictions might include characters like Gandalf trying to oppose Sauron even after he has the ring. Gandalf could be devising new strategies or trying to find a way to counter the overwhelming power that Sauron now wields. Aragorn could also be a character, still trying to unite the people against Sauron's tyranny.
There is also an old and wise wizard. He guides the hero in his journey regarding the rings, using his knowledge and magic. He has a deep understanding of the power of the rings and the danger they pose. For example, he might know the history of how the rings were created and what their true purpose is. He tries to ensure that the power of the rings is not misused by the wrong hands.
Since I don't know the exact 'the ring story', it's hard to say. But generally, there could be a finder of the ring. Maybe a young adventurer or a curious child.
I'm not sure specifically as there are many novels that could be called 'the ring novel'. But generally, there could be a protagonist who discovers the ring and gets caught up in its mystery. There might also be an antagonist who wants to use the ring's power for their own evil purposes.
It could be a young, talented wizard who has a natural gift for ring - making. Maybe Harry Potter himself might be involved, either as a customer or an inspiration for the ring - maker.
The main characters could be related to the significance of the ring. For example, if the ring belonged to a particular alien race, members of that race could be the main characters. They could be on a quest to retrieve the ring to restore their honor or to prevent some sort of catastrophe that the loss of the ring might bring about. It's also possible that the main characters are Starfleet officers who are assigned to investigate the theft of the ring, and as they dig deeper, they uncover more about the ring's importance and the motives behind the theft.
Since we don't have a lot of details about 'the ring short story', it's difficult to say exactly who the main characters are. But usually, there would be a character who discovers or owns the ring. There might also be an antagonist who wants the ring for their own purposes.
Sadako Yamamura is a main character. She is the source of the curse, a figure full of mystery and horror.