The Beast would be very confused at first. He has been the one with the beastly form for so long, and now seeing Belle in that state, he might feel a sense of guilt as if it was his fault. He would be protective of her, though, and try to help her adjust. Gaston, on the other hand, would probably see this as an opportunity to prove his 'heroism' by trying to 'save' Belle from her new form, but really he just wants to show off. Belle's father would be heartbroken and try to find a cure for his daughter, using his knowledge of inventions and books.
Well, the servants in the castle would be quite shocked. They have grown to love Belle as she is, and seeing her turn into a beast would be a big change for them. Lumière might try to cheer her up with his warm light and jokes, while Cogsworth would be more practical and start looking for ways to reverse the transformation. As for Belle herself, she would initially be horrified. But being the strong - willed character she is, she would soon start to explore her new abilities and try to find a way to turn back. And the townspeople would likely be afraid of her, which would make Belle feel sad and isolated.
Haku would likely be very protective. He has a special bond with Chihiro already, so when she turns into a spirit, he would probably try to guide her through the changes. He might teach her how to use her new spiritual powers. No - Face, on the other hand, could be either curious or envious. Since No - Face is always in search of something, seeing Chihiro transform into a spirit might make him wonder if she has found a new kind of fulfillment. The bathhouse workers might be initially shocked and a bit afraid. They are used to dealing with normal spirits, but Chihiro's transformation is something unexpected and they might worry about how it will affect the balance in the bathhouse.
Some in the kingdom might be a bit cautious at first. After all, the Beast was once feared. But as they see the child grow up with Belle's kindness and grace, they would soon come to accept and love the child. The child could also be a unifying force, bringing different parts of the kingdom closer together as everyone focuses on this new chapter in the royal family's story.
The other Pokemon in the Pokemon Center where Pikachu is usually treated would be very frightened. Nurse Joy would be at a loss as to what to do, as she has never seen Pikachu act this way. Professor Oak would be extremely concerned and would start researching immediately to find out what could have caused this sudden change in Pikachu. Team Rocket, on the other hand, might initially see this as an opportunity to capture Pikachu more easily, but they would soon realize that an evil Pikachu is far more dangerous than they expected and might end up getting in over their heads.
One possible plot could be that Belle is cursed instead of the Prince. Maybe she offends an enchantress in the forest while searching for a special flower to help her sick father. As she turns into a beast, she has to learn to control her new, wild instincts. The servants in the castle could also be transformed into various forest creatures. The story could then focus on her journey to break the curse, with the help of a kind-hearted stranger who sees beyond her beastly exterior.
Sasuke might be extremely suspicious. He has a long - standing relationship with Orochimaru, and this sudden change would make him wonder if it's a new trick. He would keep a close eye on her to see if she's still the same threat.
Luigi would probably be very surprised at first. He might be a bit confused, but then he would support his 'sister' Mario. Princess Peach would be delighted. She could finally have a female friend who has all of Mario's skills. Bowser might not even notice at first, thinking it's just another ploy by Mario. But when he realizes it's a real transformation, he might be a bit more hesitant in his attacks, not sure how to deal with this new female Mario.
Stoick would be quite surprised and a bit worried. As a father, he'd take on the responsibility of caring for his baby son. He might be a bit clumsy at first, not used to dealing with a baby again. The other Vikings would have mixed reactions. Some might find it funny, while others would be more concerned about how to reverse the situation and get their leader back to normal.
Nagisa's best friend might be really surprised at first. But then, being a true friend, would support Nagisa through this strange change. They would probably help Nagisa with things like choosing girl - appropriate clothes or dealing with any mean comments from other students.
Riku would probably be really confused at first. He might act a bit awkward around Sora, not sure how to treat this new female version of his friend. Kairi, on the other hand, could be more empathetic. She would likely help Sora adjust to being a girl, like teaching her about makeup or different hairstyles if that's relevant in the story's world.
Percy would probably be very protective. He'd be constantly worried about her safety and would do everything he could to keep her out of harm's way.