Misty could be another sidekick. She's strong - willed and has her own set of skills. She could use her water - based Pokemon to assist Ash in battles, especially against fire - type opponents. Plus, her knowledge of Pokemon and the world around them could be really helpful in their superhero adventures.
Brock could also join as a sidekick. His expertise in Pokemon care and his ability to heal and train Pokemon would be invaluable. He could take care of any injured Pokemon they come across during their adventures and also train new Pokemon that they might encounter to help in their fight against evil.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
Sakura could be a great sidekick. She has medical skills which could come in handy during investigations, like when they need to analyze strange substances or heal victims of attacks. Also, she's strong and smart, able to hold her own in a fight if they encounter resistance.
Obviously, the Legendary Pokemon would be a main character. It could be a Pokemon like Mewtwo, who is often depicted as having a complex personality. It might start off as a mysterious and powerful entity that Ash has to befriend or battle against.
His friends would be extremely shocked at first. Misty might scream and be really confused about what's happening to Ash. Brock, on the other hand, would probably try to stay calm and start thinking of ways to reverse the transformation right away. They would all be worried about how Ash is feeling in his new form and if he's in any pain. They would also be concerned about how they can still communicate with him effectively, since he's becoming a Pokemon.
It could be Team Rocket. They are always causing trouble for Ash and his Pokemon in the Pokemon series, so it's likely they would be the antagonists in this fanfiction too.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
All of Ash's Pokemon could be seen as strange and new creatures in the Power Rangers' world. The Rangers would have to learn about each Pokemon's abilities and how to work with them. Ash could hold 'Pokemon training sessions' for the Rangers, teaching them how to command the Pokemon in battles. This would not only make the battles more diverse but also strengthen the bond between Ash and the Rangers as they all learn from each other.
He could be an ex - champion who had to give up his title for some noble reason. Now that he meets Ash, he wants to see his son reach the same heights.
There's also a chance that an old enemy from Ash's past resurfaces. This enemy, who has a long - standing grudge against Ash, could find a way to use Ash's Pokemon to get revenge. They might have developed new techniques or powers over time that allow them to influence the Pokemon's loyalty.