The main characters would surely be Ash and Mewtwo. But there could also be other Pokemon that play important roles. For example, Pikachu might have a different relationship with Ash since Ash was raised by Mewtwo. And perhaps some of Mewtwo's clones could also be involved, adding more depth to the story.
The two central characters would be Ash and Mewtwo. In this fanfiction, Ash is in a rather unique situation of being raised by Mewtwo. Ash is a well - known Pokémon trainer in the Pokémon franchise, and Mewtwo is a very powerful and complex Pokémon. There could also be other Pokémon that play important roles, perhaps Pokémon that are friends or foes to Ash as he grows up under Mewtwo's care. But at the heart of it, it's Ash and Mewtwo's relationship that drives the story.
Obviously, Ash and Arceus are the main characters. Ash is the one being raised, and Arceus is the one doing the raising. But there could also be other Pokémon that are important in Ash's upbringing, like the Pokémon that Arceus assigns to look after Ash or be his companions during his growth.
Definitely his adoptive father, Giovanni. Also, the members of Team Rocket like Jessie and James would probably have a different relationship with Ash. They might see him as a sort of prodigy or future leader. And his original friends like Misty and Brock might cross paths with him, which would lead to some very tense and interesting interactions.
Obviously, Ash would be the main character since he's the one collapsing. His Pikachu would most likely be a key character as well. Pikachu is always by Ash's side and would probably be very concerned and try to help when Ash collapses.
There might also be other Pokémon that interact with Ash and Lucario in the fanfiction. They could be friends, rivals, or just part of the Pokémon world that Ash and Lucario encounter during their adventures. However, Ash and Lucario are the central focus of this particular fanfiction.
In this kind of fanfiction, it might depict Ash having unique powers or a special bond with Mewtwo. Maybe Ash grows up with a different perspective on Pokemon battles, influenced by Mewtwo's intelligence and strength.
The main characters would surely include Ash. In Pokemon Red, the rival character, Gary (or whatever name you choose for the rival in the game), could also play a major role. And of course, the various gym leaders from Pokemon Red. They could have different encounters with Ash, some might be friendly and help him, while others could be more challenging opponents. Also, Professor Oak might be involved, as he is connected to both Ash's world and the world of Pokemon Red in some ways.
The main character is surely Ash. He is the one who is being betrayed. Mew is another important one. It might be that Mew is the cause, directly or indirectly, of Ash's betrayal. Then, Ash's Pokémon partners are likely to be main characters too. They might be confused or forced to side against Ash. And there could be some antagonists in the story, perhaps some evil trainers or organizations who are behind the plot to make Ash look bad and get betrayed.
Obviously, Ash would be one of the main characters, since he's the one turning into Victini. Pikachu would also be important as Ash's most loyal Pokémon. And Victini itself could be a key character, either as a part of Ash now or as an entity that helps Ash understand his new form.
Gary Oak is a likely character. He has always been a rival to Ash and in fanfiction, he might betray Ash out of jealousy or a desire to be the ultimate Pokémon trainer. Another could be Misty. Maybe she gets influenced by some external force or her own insecurities and turns against Ash. Brock could also be a candidate in some stories. If he falls in love with a girl who has a vendetta against Ash, he might be forced to choose between his loyalty to Ash and his new love interest.