Another possibility is Bulbasaur. It has always been a loyal and steady companion to Ash. During the trip, it could see how far they've come together and confess its gratitude for all the experiences and growth they've shared.
Well, in such a fanfiction, perhaps Pikachu might be on this pokemon trip and confess its deep friendship and loyalty to Ash. It could say how much Ash means to it and how their adventures together have been the most important part of its life.
Pikachu is a must - include. It's Ash's most iconic Pokemon and always adds a lot of charm and power to the story. Brock could also be great as he has a lot of knowledge about Pokemon and can offer valuable advice.
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.
Ash is a well - known Pokémon trainer, always eager to catch new Pokémon and battle strong opponents. He has a kind heart and is very passionate about Pokémon. Dawn is another trainer, who is often shown to be determined and has her own unique style of training. In the 'dawn confesses to ash fanfiction', their relationship takes a turn towards the romantic as Dawn decides to be honest about her feelings for Ash.
Maybe Dawn could be Ash's date. They have a great friendship and have journeyed together. Their knowledge of Pokemon contests and battles could be a fun part of their date during the trip. Also, Anabel from the Battle Frontier is a possibility. She's a strong trainer, and Ash has had interactions with her. A date with her during a trip to the Battle Frontier or another region could bring some new and exciting dynamics to the story.
Brock could be an interesting secondary character. His knowledge of Pokemon and his role as a sort of mentor figure could add depth to the story. He might give Ash and Trip advice on Pokemon care or training.
Another possibility is that it could be an old acquaintance who has changed for the worse. They might have been jealous of Ash and Lyra's success or popularity in the Pokémon world. This person could use their knowledge of Ash and Lyra's habits and weaknesses to betray them. It could also be a group of people rather than just an individual, like a team of thieves or poachers who see Ash and Lyra as obstacles to their illegal activities.
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.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
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.