For Pikachu, it might be very confused. Pikachu is very close to Ash and would sense the change in Ash's mood. It could start to be more protective of Ash and even be a bit hostile towards Serena. This would add an interesting dynamic to the story as Pikachu is usually friendly.
Brock would be more understanding. He's always been interested in girls, but he also values friendship. So he would probably offer his support to Ash, while also maybe giving some advice on how to act more 'ladylike' if that's what the situation calls for. As for the other Pokemon in Ash's team, they might not fully understand the concept of crossdressing, but they would sense that Ash is still their trainer and would protect him (or her in this case) as usual.
Their relationship would likely grow closer. At the dance party, Ash might be a bit clumsy at first, and Serena would gently guide him, which would create some sweet and tender moments between them. They would share laughter and maybe some embarrassed blushes as they interact more intimately during the dance, deepening their bond.
Their relationship could develop with them learning more about each other's dreams. Ash might support Serena in her new cooking business that focuses on Pokemon - friendly foods. And Serena could encourage Ash to take part in more international Pokemon battles.
If Ash was in Serena's body, his relationship with her friends like Bonnie and Clemont would change. Bonnie might be a bit confused as Ash's more boyish and impulsive nature shows through Serena's body. Clemont might be surprised at some of the battle - like decisions Ash makes when dealing with things like helping around the lab. On the other side, if Serena was in Ash's body, her relationship with Ash's rivals like Gary would be different. Gary might be intrigued by the more refined and composed 'Ash' he now sees.
One idea could be that Serena's 'cheating' might be a misunderstanding. Maybe she was seen with another trainer in a compromising position, but in reality, they were just discussing a very important Pokemon training technique that Ash didn't know about at the time.
At first, Serena might be a bit wary of Kaa. But as they face challenges like dealing with a group of unruly Pokemon or finding food in a scarce area, they start to understand each other better. Kaa might show Serena some hidden secrets of the Pokemon world, and Serena could in turn introduce Kaa to new Pokemon species, gradually strengthening their bond.
In the Alola setting, their relationship could develop as they explore the islands together. They might share more personal stories while sitting on a beautiful Alola beach, watching the sunset with their Pokémon by their side. This could bring them closer emotionally.
In these fanfictions, Ash often develops more complex emotions towards Serena and other characters in the harem. He might become more aware of his feelings beyond just friendship. For example, he could start to notice Serena's unique qualities more deeply. Serena, on the other hand, may grow in confidence as she is part of a group that includes Ash. She might become more assertive in expressing her feelings for him.
During time travel, their relationship could take a new turn. They might discover new sides of each other. If they travel to a time where Pokémon battles were different, Serena could see Ash's adaptability and grow more attracted to him. On the other hand, Ash could see Serena's quick thinking in handling the unfamiliar situations in the past or future, which would make him appreciate her more. It could be a journey of mutual discovery and growing affection.
Serena's character development in fanfiction is a journey of self - discovery. She discovers her own strengths and weaknesses as a Pokemon trainer. With Shauna by her side, she learns to be more independent. Shauna, for her part, grows in her understanding of different Pokemon types and training methods. Their interactions also lead to emotional development. They learn to support each other through tough times, like when Serena loses a big battle or Shauna has trouble catching a particular Pokemon. This mutual support system helps them both become more well - rounded characters in the fanfiction world.