Some characters might be shocked. If Captain Ginyu changes into a body that is much weaker or stronger than his original one, his allies or enemies will have different reactions. His enemies may underestimate him if he's in a weaker body at first, only to be surprised later. His allies might be confused about how to work with him in a new form.
Some characters may be curious. They might want to study how the body change actually works. They could try to find a way to either replicate it for themselves or find a weakness in it. This curiosity could drive them to interact with Captain Ginyu in his new form in a more scientific or investigative way.
Another interesting plot could involve the emotional aspect. Suppose he changes into a body that has a family or friends. He would then have to deal with the relationships associated with that body. He might find it difficult to fit in and pretend to be the person who originally owned the body, leading to some comical or even heart - wrenching situations.
His relationship with Bucky would be severely strained. Bucky might not believe that Cap is truly brainwashed at first and try to reach out to him, but Cap could see Bucky as a threat. With Tony Stark, there would be more conflict. Since they already had their differences, a brainwashed Cap could be much more aggressive towards Tony. And his relationship with Natasha would also change. She might be the first to suspect the brainwashing and try to find a way to save him, but Cap would be suspicious of her motives.
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.
They could be surprised. Fanfiction often takes the characters in new and unexpected directions. So characters might be shocked to see versions of themselves or Captain America that are different from what they know.
Pansy Parkinson would likely have a very different reaction. She might be more excited about the change, seeing it as an opportunity to be even more fashionable and to stand out in a different way. She could use her new appearance to gain more popularity among the students or to make a stronger statement within the Slytherin house.
For his male friends like Shikamaru and Choji, they might initially be a bit awkward around Naruto. But as time passes, they would adapt and might start to treat him more like one of the girls in their group. Naruto's relationship with the older male ninjas like Jiraiya could also change drastically. Jiraiya might not be able to train Naruto in the same way as before and might have to come up with new training methods that are more suitable for Naruto's new form. Naruto's friendships with female characters like Ino could also transform. Ino might take Naruto under her wing and teach him things like how to do proper makeup or how to style his hair in the ninja world.
Some characters might be critical. If they have a strong sense of the 'canon' Superman, they could nitpick at the details in the fanfic that don't match up. For instance, if the fanfic changes Superman's origin story too much, they might frown upon it.
They could be shocked. If the fanfic takes a really unexpected turn, like making a villain into a hero suddenly, the characters might be taken aback. It's like seeing a completely different version of their world.
Sakura, being a medic - ninja, would likely be very concerned and immediately start trying to diagnose and treat Naruto. She might be constantly by his side, using her medical skills to ease his symptoms.
Hermione would probably be very supportive. She's always been open - minded and a strong advocate for equality. She would likely help Harry with any practical aspects of the transition, like finding appropriate clothes or dealing with the different expectations in classes. Ron, on the other hand, might be a bit more confused at first. He's used to Harry as a boy, but eventually he would come around and still be a loyal friend.