The Joker would probably be both delighted and a bit confused. He's used to Batman being his nemesis, but an evil Batman would change the dynamic. He might initially see it as a new and exciting game, but then realize that an evil Batman could be a real threat to his own chaotic existence. The Joker might try to manipulate the situation to his advantage, but it could easily backfire on him.
The other members of the Bat - family, like Nightwing and Batgirl, would be in shock at first. They would initially not believe that Batman could turn evil. But once they realize it's true, they would try to stop him. They might try to find a cure if it's a case of mind control or try to reason with him if it's a psychological shift. However, they would also be conflicted because he was their mentor and friend, and they would be reluctant to use lethal force against him.
Batman would be heartbroken. He has trained Robin and sees him as a partner and almost like a son. He would first try to talk to Robin, to find out what made him turn evil. He would hope that he could reason with Robin and bring him back to the good side.
Another plot could be that he is exposed to some sort of mind - controlling substance or technology. The enemy could be using it to turn him against Gotham. Under this influence, he starts to see his friends as foes. He could begin to attack the Bat - family and other superhero allies. His actions would be erratic and violent, and the city would be in chaos as they try to figure out what happened to their hero and how to stop him.
The villains would have a mixed reaction. Some of the more power - hungry ones might see this as an opportunity to join forces with Saitama and cause even more chaos. However, there would also be villains who are afraid of Saitama's power, even if he's on their side now. They would be worried that he could turn on them at any moment. Overall, Saitama turning evil would completely disrupt the existing power dynamics in the One Punch Man world.
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 twist could be that Robin is influenced by a powerful and evil villain. Maybe the villain uses some kind of mind - control device or magic on Robin. For example, the Joker could find a way to make Robin see Batman as the enemy. Robin might then start attacking Batman and helping the Joker with his evil plans.
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.
The other gumballs in the gumball machine might be very confused. They've never seen such a thing happen before. Some might be scared and think it's some sort of curse. The humans who find the girl - gumball could be initially shocked. But some kind - hearted ones might take her in and try to help her adjust. If there are any pets in the story, they might sniff her curiously, not really understanding what she is but sensing that she's different.
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.
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.
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.