Maybe an enemy ninja from another village who has caught wind of Naruto being a secret ANBU. This antagonist would then try to use this information to their advantage, perhaps by blackmailing Naruto or using it to create chaos within Konoha. They might also try to steal ANBU secrets that Naruto has access to.
It could be a rogue ANBU member who has gone bad and is trying to expose Naruto's secret identity for some evil purpose.
One option for the main antagonist could be a secret organization that has infiltrated Konoha. They target the Anbu prodigy because they fear the prodigy's abilities could expose their existence and plans. This organization might have members with unique and dangerous abilities themselves. They would constantly send assassins after the prodigy and try to create chaos in Konoha to keep the Anbu and the prodigy distracted while they carry out their real objectives.
It could be an evil shinobi who wants to capture the Kitsune Anbu for their unique powers. Maybe they want to use those powers to take over the Hidden Leaf Village. Another option is a rogue Anbu member who has turned against the village and sees the Kitsune Anbu as a threat to their own plans.
Perhaps a powerful shinobi from another village who has a special grudge against the Leaf's ANBU. He targets Kakashi and Naruto specifically because of their reputation within the ANBU. He has unique abilities that pose a great challenge to them, like the ability to nullify ninjutsu for short periods.
The main antagonist could be an old - school ninja who believes in the traditional and often harsh ways of the ANBU. They see Naruto's actions as weakening the village and set out to oppose him at every turn, trying to bring back the old ANBU regime.
One potential main antagonist could be an older, power - hungry shinobi who has infiltrated the Anbu in the past. This shinobi could be jealous of Naruto's sudden rise in the Anbu (due to his future knowledge) and sees him as a threat. He then starts to plot against Naruto and the Anbu's new - found positive direction. Or perhaps it's a corrupt politician in the village who uses the Anbu as a tool and doesn't want Naruto to change the Anbu's loyalty from him to the village as a whole.
It could be a rogue Anbu member. This person might be jealous of Naruto's power and popularity within the Anbu. So, they spread false rumors about Naruto which lead to his decision to leave the village. This antagonist would then try to further harm Naruto while he's outside the village, using their own Anbu skills against him.
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.
It could be Orochimaru. He has always been interested in powerful jutsus and Naruto's unique power as a Jinchuriki. He might have developed a new jutsu to make everyone forget Naruto so that he can capture him without interference for his own experiments.
The main characters are Naruto and Hinata. Naruto is known for his never - give - up attitude and his dream of becoming Hokage. Hinata, on the other hand, has a long - standing admiration for Naruto and her own unique skills in the Hyuga clan. In this fanfiction, they are likely to be the central focus as they take on the challenges within the ANBU.
There's also a chance that the main character could be a completely new character from a minor clan in the Naruto universe. This character could have some hidden bloodline limit that makes them a prodigy in the ANBU. They might start as an underdog but quickly rise through the ranks due to their exceptional skills and intelligence.