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.
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.
One possible plot could be Pikachu getting influenced by a dark and powerful Pokemon. For example, a Darkrai might use its nightmare - inducing powers on Pikachu. This could make Pikachu start to attack other Pokemon and trainers without mercy. Maybe Pikachu would first target its own trainer, Ash, as it would be the most shocking turn of events. This would create a lot of chaos in the Pokemon world and force Ash and his other Pokemon friends to try and figure out how to save Pikachu from this evil state.
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.
Ash would be extremely worried. He'd do everything he could to make Pikachu feel better, like staying up all night to take care of it. Misty might offer some practical advice, like making sure Pikachu stays warm. Brock would probably be ready to cook some special food for Pikachu to help it regain its strength.
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.
Ash would be completely devastated. Pikachu has been his closest companion since the start of his journey. He would probably go into a state of shock at first, unable to believe that Pikachu was gone.
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.
His friends would be very shocked at first. Misty might be a bit worried and try to find a way to turn him back quickly. Brock, on the other hand, would be more curious and start observing Ash - as - Pikachu to see how his behavior has changed. His Pokémon friends would also be confused. Pikachu himself might be both excited to have a 'Pikachu - like' friend but also a bit sad that his trainer is in this strange state.
Obviously, Evil Pikachu is a main character. There might also be trainers who are trying to either stop it or control it. Maybe some other Pokémon that are allied with or against the Evil Pikachu could also be considered main characters.
Zazu would be flustered. He's used to reporting to the grown - up Simba and giving advice. With Simba as a baby, he would be at a loss for a while. He would try to maintain order in the pride but would also be constantly worried about Simba's safety. The hyenas, if they got wind of this, would see it as a chance to cause chaos. They might try to attack the pride while they think Simba is vulnerable. But the pride, united by their love for Simba (even in his baby form), would be ready to defend their king and their territory.