Sasuke would be very conflicted. On one hand, he has his own goals, but on the other, Naruto is his friend. He might initially try to fight Naruto to stop him from doing harm, but then he would focus on finding a way to break the brainwashing.
Sakura would be heartbroken. She would see Naruto as a victim and would do everything she could to bring him back. She might use her medical ninjutsu knowledge to try to find a cure for the brainwashing, and she would also be on the front line to protect others from a brainwashed Naruto.
Naruto's reaction could be complex. He might be sad and disappointed, but he also has a lot of love in his heart. He could try to forgive his teammates, believing that there must be some misunderstanding. However, if they continue to oppose him, his reaction would turn more towards self - preservation and growth. He would focus on his own development, both as a ninja and as a person. He would use this betrayal as a motivation to become stronger, all while still hoping that one day he can reconcile with Team 7.
His friends would be horrified. Sasuke, for instance, who has known Naruto for a long time, would be shocked at this transformation. He might initially try to reason with Naruto, thinking that there's still some good left in him. But as Naruto's psychopathic behavior continues, Sasuke would be forced to consider him an enemy and take action to stop him if necessary.
Naruto is a likely candidate. Given his importance in the series, a plot where he is brainwashed can create a huge impact. His powerful chakra and his role as the main character mean that if he is brainwashed, it can change the entire dynamic of the story. The enemy could use his strength against his own friends and village.
The Hyuga clan members would be very confused by 'Hanabi' (really Naruto). They would notice her (his) un - Hyuga - like behavior and attitude. For example, Naruto's loud and friendly nature would be a stark contrast to Hanabi's more reserved Hyuga self. As for Naruto's friends, they would be puzzled by 'Naruto' (actually Hanabi). They would wonder why he (she) seems a bit different in combat and decision - making.
Tsunade would probably be a bit surprised at first. She might be a bit protective of Shizune and would want to make sure Naruto is serious about her. Shikamaru might be the first to notice and just shrug it off as another strange relationship in the village. Sakura might be a bit jealous, not because she has feelings for Naruto in that way anymore, but because she's used to being one of the main girls in Naruto's life. And Kakashi? He'd probably just give Naruto some knowing looks and maybe some advice about relationships if Naruto came to him for help.
Yes, there could be such fanfictions. Fanfiction allows for a wide range of creative interpretations. Some writers might deviate from the original characters' traits and create a story where Hinata cheats on Naruto. But this is not in line with the general perception of their relationship in the original series.
If Naruto was an Ajin in a fanfiction, the characters in Naruto would have a mix of reactions. Hinata would be concerned for Naruto's well - being. She might be a bit scared at first because of his new and strange abilities, but still stand by him. The villagers, who have had a rocky relationship with Naruto in the past, might be afraid of him all over again. They could see his Ajin powers as a threat. On the other hand, Naruto's close friends like Shikamaru would try to understand what's going on and help Naruto control his powers if they can.
Naruto would probably be very curious. He'd want to know all about the bunnygirls, like where they come from and what their powers are. He might be a bit shocked at first, seeing as they're not like the people he's used to in the ninja world, but his adventurous spirit would quickly take over and he'd start making friends with them.
Sakura would be extremely worried. She would probably try to gather the other members of Team 7, like Sasuke and Kakashi, to go after them. Sasuke might be a bit more hesitant at first, but his loyalty to Naruto would eventually make him join the search. Kakashi, on the other hand, would be more strategic. He would try to figure out Minato's motives first and then plan the rescue mission accordingly.
Sakura might be a bit surprised at first, but she would ultimately be happy for Naruto. She has grown past her initial crush on him and would see that Pakura makes him happy. Kakashi would probably be very supportive, seeing it as another step in Naruto's growth and a positive development for the ninja world. Sasuke might be more reserved in his reaction, but deep down he would understand Naruto's choice and respect it.