His friends like May and Max would be really surprised. May might giggle a bit at first but then feel sorry for Ash. She could try to find some cute Pokemon - themed clothes for him. Max would be curious about how this happened and would want to help find a way to reverse it. Professor Oak would be very concerned and would immediately start researching what could have caused this strange transformation, while also trying to make sure Ash is comfortable.
Pikachu would probably be very protective. It might zap anyone who tries to make fun of Ash. Misty would likely be shocked at first, but then she'd start taking care of him like an older sister, making sure he's clean and fed. Brock, being the nurturing type, would probably be the one to find practical solutions for taking care of baby - Ash, like finding proper diapers in the Pokemon world.
Pikachu might be a bit confused at first. He's used to Ash's adventures mainly being about Pokémon battles and training. But he would soon get used to it and might even play a role in making the 'date' more fun. For example, using his electric attacks to create some light shows for them.
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.
Ash would probably be very shocked and hurt. Pokemon are like his family, and for Misty to steal one would go against everything he stands for.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
Well, a plot could be where Ash travels to a strange region where there is a custom that all trainers start their journey as babies in diapers for some unique reason related to the local Pokemon lore. Ash has to adapt to this new way of starting his adventure while also facing the typical challenges of training Pokemon. Or perhaps there's a time - travel aspect where Ash goes back to his own baby days in diapers and has to use his future knowledge to help his younger self and the Pokemon in that time period.
His friends like Pikachu would be very worried. Pikachu would probably stay by Ash's side, trying to use its electric shocks gently to revive him. Misty might scold Ash for being so reckless but also show concern by getting water to cool him down. Brock would immediately start checking Ash's vital signs and try to figure out what went wrong.
I think Ash would be angry as well as hurt. He's a passionate trainer and his friends mean a great deal to him. He might challenge May in a Pokémon battle, not to defeat her but to make her see how wrong her betrayal is. And during the battle, he would keep asking her to come back to their old friendship.
Ash would be extremely hurt. He's a very loyal person, and having someone he cares about like Lillie betray him would be a huge blow. He might initially go into denial, not believing that Lillie could do such a thing.
Ash's Pokemon would likely have mixed reactions. Some like Bulbasaur, being the gentle and observant one, would be quick to accept Sabrina. It might even enjoy the new psychic powers that Sabrina could bring into their group. However, Squirtle might be a bit mischievous at first, trying to test Sabrina's patience. For Sabrina's Pokemon, Alakazam, being highly intelligent, would sense Ash's good intentions towards Sabrina and would support the relationship. Her other Pokemon would follow Alakazam's lead.
One way to find popular 'ash in diapers fanfiction' could be to search in specialized fanfiction forums or communities. These places might have users who are interested in such unique concepts and could recommend some stories. But keep in mind that this is a very niche area of fanfiction.