The other characters' reactions would vary greatly. For instance, Gary, who has always had a bit of a rivalry with Ash, would be shocked. He might initially be in denial but then realize the depth of Ash's selflessness. Professor Oak would be extremely sad and would see Ash as a true hero. The Gym Leaders that Ash has faced and befriended would hold commemorative events in their Gyms. They would talk about Ash's courage and how his sacrifice has left a huge mark on the Pokémon world. His sacrifice would also inspire younger trainers to be more like him.
In the fanfiction, when Ash sacrifices himself for Mew, Tracey would be filled with disbelief. He would feel guilty that he couldn't do anything to stop it. May and Dawn, who look up to Ash, would be heartbroken and would vow to carry on Ash's legacy. They would try to be stronger and more selfless in their own Pokémon training journeys. The Pokémon League officials might create a special memorial for Ash. And the other trainers around the world would hear about his sacrifice and start to tell stories about him, making Ash a symbol of ultimate sacrifice and heroism in the Pokémon community.
One possible plot could be that in a world filled with dark forces threatening all Pokémon, Ash discovers a way to save Mew who holds the key to restoring balance. Ash, being the selfless hero, decides to sacrifice himself to protect Mew from an oncoming, unstoppable attack. This act of sacrifice then becomes a legend among Pokémon trainers, inspiring others to be brave and selfless.
Pikachu would most likely be there. Since Pikachu is Ash's closest companion, it would be involved in any story related to Ash. Maybe Pikachu could also have some sort of interaction with Mew, like being a bit jealous at first of Ash's new connection with Mew.
At first, his friends might not believe it's Ash. But when he starts to show his usual personality traits, like his determination and kindness, they would start to accept it. They would then start to think about how they can use Ash's new form to their advantage. For example, they could use his ability to fly as Mew to reach places they couldn't before, or his psychic powers to sense danger from afar.
In fanfiction, the relationship between Ash and Female Mew can be very interesting. It often involves unique storylines where Ash might encounter Mew in a different context than in the original Pokémon series. For example, the fanfiction could explore how Ash's adventures change when he has a special bond with Female Mew. Maybe they go on a journey together to discover new Pokémon or face a powerful enemy that only they can defeat as a team. This relationship in fanfiction can bring out different aspects of Ash's character, such as his compassion and determination in a new light.
The characters' reactions would be complex. For instance, Iruka Sensei, who was like a father figure to Naruto, would be inconsolable. He has watched Naruto grow from a troublemaker to a hero, and this sacrifice would be too much to bear. Shikamaru, being Naruto's friend, would feel a mix of admiration and sadness. Admiration for Naruto's selfless act but sad that he's gone. The kids in the village who looked up to Naruto would be inspired in a way. They would want to be like Naruto and protect the things they love, even if it means sacrificing themselves.
Hinata would be completely devastated. She has always been deeply in love with Naruto, and his sacrifice would leave her heartbroken. She would still be strong for Boruto, though, and try to support him in any way she could.
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.
Perhaps it would be a combination of forces that save Ash. His own Pokémon, led by Pikachu, could start the rescue mission. Along the way, they might encounter some friendly Pokémon trainers who offer their help. And there could also be some unexpected help from a Pokémon that Mew least expects. This Pokémon could have some sort of connection to Mew and be able to convince it to let Ash go or help Ash escape while Mew is distracted.
Often, it starts with Ash being in awe of Mew's rarity and power. Mew, seeing Ash's pure heart, decides to approach him. Then they start to trust each other more as they face challenges together, like saving a group of Pokemon from a natural disaster.
One common theme is friendship. Ash and Mew often form a special bond in these fanfictions, like in 'The Friendship of Ash and Mew' where they rely on each other to overcome difficulties. Another theme is adventure. They go on various quests together, exploring new places and encountering different Pokémon.