Who could be the main antagonist in a 'Naruto has a fox brother' fanfiction?

Another possibility is that there is a group of anti - jinchuriki people who see Naruto as a threat because of his connection to the giant fox. They might try to kill Naruto to get rid of the 'giant fox problem'. Their leader could be the main antagonist, someone who has a deep - seated hatred for anything related to the tailed beasts.

It could be a powerful ninja - demon hybrid. This antagonist could be jealous of the bond between Naruto and Inuyasha. He wants to use their combined power for his own evil purposes. So, he tries to turn them against each other first by spreading false rumors and then by attacking their loved ones.

It could be a rogue fox - clan member who wants to use the power of the Nine - Tailed Fox inside Naruto for his own evil purposes. He might try to manipulate Naruto or directly attack the Fox Clan to get what he wants.

The main antagonist could be one of the village elders. They might see Naruto's fox - summoning abilities as too dangerous and banish him out of fear. Throughout the fanfiction, they could try to prevent Naruto from coming back to the village, fearing that he'll seek revenge or disrupt the power balance they've established.

Maybe the main antagonist is an ancient spirit that has been awakened by Naruto's experiments with transformation. The spirit doesn't like Naruto's interference with nature's balance and decides to punish him by making him turn into a fox with the strange 'lemon' aspect. This spirit then tries to keep Naruto in this form permanently and drive him mad, so that Naruto can no longer be a threat to the spirit's plans of ruling over the ninja world.

Well, it could be about the fox brother having special powers. Maybe he can communicate with Naruto in a unique way, like through telepathy. They might go on adventures together, facing off against enemies that Naruto couldn't handle alone. The fox brother could also have a connection to Naruto's own inner fox spirit, and they could work together to control that power better.

It could be Orochimaru. He is always interested in new and powerful weapons. With Naruto having hidden blades, Orochimaru might see it as a threat and try to take the blades for himself, leading to a conflict.

It could be a rogue ninja who was once part of the same clan that the hidden blade belongs to. He wants the blade back to gain ultimate power and is willing to do anything to get it from Naruto.

It could be a rogue ninja who also has a rare bloodline and sees Naruto as a threat to his own power and status. This antagonist might want to steal Naruto's bloodline or simply eliminate him to be the only one with such a special ability.

Perhaps the main antagonist is a dark figure from within Konoha itself. This person doesn't want Naruto's family to be reunited because it could expose some dark secrets about their own actions in the past. They would do anything to keep the sister hidden and Naruto in the dark, even if it means causing harm to both of them.