If Ash marries Sabrina in a fanfiction, his Pokemon would react in different ways. Pikachu, being Ash's closest partner, would be the first to welcome Sabrina. It would probably jump on her shoulder and give her little electric shocks of excitement. His flying type Pokemon like Pidgeot might be happy as they see Sabrina as someone who can help Ash in a different way. Sabrina's Pokemon, especially her psychic ones, would initially be curious about Ash. But as they see how much Ash and Sabrina care for each other, they would be happy to be part of this new family of trainers and Pokemon.
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 possible storyline could be that Ash, after many adventures, returns to Saffron City where Sabrina is. Their relationship could start from Ash challenging her gym again but this time they see each other in a different light. Maybe Pikachu plays a role in bringing them closer, like it gets lost in Sabrina's psychic maze and Ash has to work with Sabrina to find it. This shared experience could be the start of their romantic journey.
Ash might be initially shocked. He has always seen Sabrina as an acquaintance or even a friend in some cases. So when she betrays him, he'd be taken aback. For instance, he could freeze for a moment, unable to believe what has just happened.
His Pokemon might be a bit confused at first. Pikachu, for example, is used to their adventures on the road. But they'll soon adapt. They could make new Pokemon friends at the school too. And they'll be there to support Ash during his school challenges, like in battles or tests.
One interesting story could be about Ash and Sabrina teaming up to face a powerful, new, and mysterious Pokemon. They might use their unique skills and bond to overcome the challenges.
One common theme is the power of friendship evolving into love. They start as friends or acquaintances in the Pokemon world and as they interact more, feelings of love develop. Another theme could be overcoming differences. Ash is more of an energetic and sometimes reckless trainer while Sabrina is calm and powerful. They have to overcome these differences to be together.
Yes, there are. For example, some fanfictions might depict their adventures together where their relationship gradually develops. They could be on a journey to discover new Pokemon and during that time, Ash starts to see Sabrina in a different light, and Sabrina also becomes more open to Ash.
It really depends on the story. In some fanfictions, Sabrina might be more dominant. Given her psychic powers, she can control situations easily. For example, she could use her powers to influence the outcome of battles or make quick decisions in dangerous situations.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
Pikachu might be very confused at first. When it sees 'Ash' (really May in Ash's body) acting differently, it could be hesitant to follow commands as usual. For May's Pokemon, they might be surprised to see 'May' (Ash in May's body) suddenly having a more battle - oriented attitude. For example, May's Beautifly might be confused when 'May' wants to train it for battles instead of focusing on its appearance for Contests.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.