Definitely Dawn. And then there are her traveling companions. May, if she were to make an appearance in a crossover type of fanfiction, could be involved. May might be curious about Dawn's experiences with tickling in the Sinnoh region. Also, Nurse Joy could potentially be part of the story. Maybe Dawn gets tickled by a Pokemon while at the Pokemon Center and Nurse Joy has to deal with the resulting chaos and laughter.
Ash and Serena are often the main characters. Their relationship in the Pokemon anime has a lot of potential for amorshipping fanfiction. Ash's determination and Serena's grace make for an interesting dynamic in the stories.
Often, young and ambitious trainers are involved. They might be curious about Pokemon abilities and conduct their own little experiments. Scientists also play a big role. They usually have the resources and knowledge to carry out more complex experiments on Pokemon in a more formal setting.
Riley Matthews is often involved. She has a bubbly personality, so it's easy to imagine her being part of tickle - related scenarios, either as the one being tickled or tickling others in a friendly way.
In some 'pokemon champion ash fanfiction', Gary could be an important character. He has always had a competitive relationship with Ash, so in stories where Ash is a champion, Gary might try to challenge him or be involved in some sort of plot related to Ash's championship. Another character could be Serena, especially if the fanfiction focuses on the romantic aspect of Ash's life along with his championship journey.
In many fanfictions, Gary Oak can also be a significant character. Since Ash is smart in these stories, there could be a rivalry between him and Gary where they constantly outwit each other in Pokemon battles and in their knowledge of Pokemon lore. Another character could be Professor Oak, who might be amazed at Ash's new intelligence and give him special tasks or Pokemon to train.
Kenzi, Bo's sassy best friend, is almost always a key character in 'lost girl tickle fanfic'. Her sharp tongue and quick thinking can add a lot of humor to tickle - related scenarios, like coming up with creative ways to start or stop a tickle fight.
Obviously, Ash is a main character. He is usually at the center of the story, going on his adventures in the Alola region.
In Pokemon birth fanfiction, Misty can be a popular character. She has a strong connection with water - type Pokemon, and she could be involved in the birth of a water - type Pokemon, perhaps using her knowledge of water to create a suitable environment. Professor Oak is also a character that often shows up. His vast knowledge of Pokemon would make him a valuable asset during a Pokemon birth. And Pikachu, as Ash's most famous Pokemon, might also be part of the story, either as a helper or just being curious about the new - born Pokemon.
Ron Weasley is another likely character. He's always up for some fun and mischief, so he could be the one who discovers a new and powerful version of the tickle spell and tries it out on his friends, leading to all sorts of hilarious adventures in the fanfic.
Misty and Brock are also main characters in many of these fanfictions. After Ash's sacrifice, they might be the ones to comfort Pikachu and the other Pokémon. They could also take on Ash's unfinished tasks, like continuing to explore the Pokémon world, helping other trainers, and fighting against any emerging threats. Their friendship with Ash gives them the motivation to keep going in his absence.