Sasuke would be extremely suspicious. He has always seen Naruto as his rival and a symbol of the village's hope. If Naruto starts acting like Pein, Sasuke would think it's some kind of trick at first. He might even be ready to fight Naruto to figure out what's really going on.
Kakashi would be very concerned. He has mentored Naruto and has seen his growth. If Naruto turns into someone like Pein, Kakashi would try to talk to him, to understand what led to this change. He would probably try to remind Naruto of his true values and the people who care about him, hoping to bring him back to his senses.
Sakura would be heartbroken. She's always had feelings for Naruto, and seeing him insane would be a huge shock. She'd likely try to use her medical skills to find a cure for whatever is ailing Naruto's mind. She might also try to get help from Tsunade, hoping that the former Hokage's knowledge could save Naruto.
Sakura would be worried sick. She'd try to figure out what happened to Naruto to make him act this way. She might try to talk to him, use her medical ninjutsu to check if he's under some kind of genjutsu or has some other problem. She'd be very vocal about her concerns and would try to get the other villagers to help bring Naruto back to his normal self.
All Might would probably be very supportive. He's always been about the spirit of heroism, not gender. He'd encourage Izuku (as a girl) to keep training hard. Bakugo might initially be shocked and then try to act all tough, but deep down he might be a bit confused about his feelings towards her. Ochaco would be excited to have another girl around and might become even closer friends with Izuku.
Some characters might be very surprised at first. For example, Asuna could be initially shocked but then become very protective of Kirito as a girl. Klein might make some teasing remarks but still be a good friend. And the villains might see her as an easier target or a new kind of threat depending on how they view female characters in general.
The villagers in the Hidden Sound Village would likely be in awe at first. Some might be skeptical, but as Naruto starts to show his leadership and use his godlike powers for the good of the village, they would come to respect him. In Konoha, his old friends would be proud of him, but also a bit sad that he's leading another village. Villains in the Naruto universe would probably be more cautious, knowing that Naruto's power as Otokage could be a major threat to their plans.
Sasuke would likely be very suspicious. He has always had a complex relationship with Naruto, and this new, twisted Naruto would be a huge red flag for him. He might initially think it's a trick or some new form of power Naruto is trying to master, but as he realizes it's a real change, he would be on guard. Sakura would be worried and try to find a way to bring Naruto back to his old self. She has always cared about Naruto and would not be able to stand seeing him like this.
The Akatsuki members, if any are still around, would have diverse reactions. Some might see it as a weakness in Pein, while others might be curious about how this new family dynamic could change the balance of power in the shinobi world. Hinata would be shyly supportive of Sakura. She would understand the importance of love and family, and she would hope that Sakura finds happiness, even if she's a bit intimidated by Pein's past actions.
Sakura might be initially shocked and then start laughing uncontrollably. She would probably tease Naruto about it for a while. Sasuke, on the other hand, would likely just give a slightly raised eyebrow and then act as if it's not a big deal, but deep down he might be a bit amused. Kakashi would probably make a dry joke about it.
Sakura might be initially shocked but then she would be determined to help Naruto find out what's wrong. She could start researching in the hospital's library for any medical or historical records related to such a phenomenon.
Kakashi would likely be amused by the whole situation. He might give Naruto some tips on how to blend in better as a girl, using his own knowledge of disguise techniques. Iruka would be more concerned about the reasons behind Naruto dressing as a girl and would make sure Naruto is comfortable with it. He would also be there to support Naruto if any of the other villagers had a negative reaction.