Many fanfictions pair Ash, in his reincarnated state, with new characters as well. Sometimes there are powerful Pokémon trainers or even mysterious characters from different dimensions. These new pairings can introduce fresh storylines and dynamics, like the new character guiding Ash through his new, reincarnated journey or being in conflict with him over different goals.
Clemont is also often paired with Ash in interesting ways in fanfiction. They could be on a scientific exploration in Kalos, where Clemont's inventions play a crucial role in their adventures. Their friendship might be developed in a different way, like they have to rely on each other more than in the original series to overcome some big challenges related to Pokémon and the Kalos environment.
Mewtwo is also a common Legendary Pokémon in Ash - related fanfiction. Mewtwo's complex personality and its powerful psychic abilities make for interesting interactions with Ash. There can be stories about Mewtwo respecting Ash's bond with his Pokémon and joining him on a journey, or even testing Ash's worthiness.
Naruto can also be paired with Sakura in these fanfictions. Since they are such close friends in the original, in a reincarnation scenario, their friendship could potentially turn into something more romantic. They could support each other in different ways than in their past lives.
One popular Pokegirl could be Misty. She has been with Ash for a long time and they have a great dynamic. Their bickering could easily turn into a more romantic relationship in fanfiction. Another is Serena. She has a clear crush on Ash in the anime, so it would be natural to explore a deeper relationship between them in fanfiction as a Pokegirl - like scenario.
One common character is of course Ash himself. Another could be Pikachu, as it is Ash's most loyal Pokémon. There might also be Misty or Brock, as they are Ash's long - time friends and could play a role in the adoption story, either being supportive or having doubts about the new situation.
There are also often appearances from various Gym Leaders. They can serve as either allies or rivals to Ash. For instance, a Gym Leader might challenge Ash to a battle to test his newfound power, or they could join forces with him to take on a greater evil that threatens the Pokemon world.
Pikachu is definitely a common character. It's Ash's most loyal partner and is always by his side during his adventures. Serena is also often present. She has her own dreams and goals related to Pokemon and often travels with Ash, providing support and sometimes even competing with him.
Obviously, Ash is a common character. His friends like Brock and Misty might also make appearances. They could offer their own skills and knowledge to help Ash in his ranger adventures.
Some fanfictions might also include new characters that Ash meets on his return journey. These could be trainers with unique Pokemon or mysterious characters who have a connection to the events that led to Ash's return. For example, a young trainer who idolizes Ash and wants to learn from him, or a character who has information about a powerful Pokemon that Ash is interested in.
Obviously, Ash is a central character since it's about him vomiting. Pikachu is likely to be there too, probably looking worried about Ash. Maybe Misty would be in the story as well, scolding Ash for getting into a situation that made him vomit or trying to take care of him.