Ash would be terrified. He is facing a group of Spearow without his most reliable Pokémon by his side. He would also feel disappointed in Pikachu. After all the adventures they've had together, he never expected Pikachu to leave him in such a dangerous situation. He might start to doubt their friendship and partnership.
At first, Ash would be in disbelief. He can't believe that Pikachu is actually leaving him. Then, as the Spearow approach, he would feel a mix of anger and fear. Anger towards Pikachu for leaving and fear because he has to deal with the Spearow on his own. This experience could change his view of Pikachu and their relationship forever.
Perhaps Pikachu is lured away by something more appealing. There could be a rare and powerful Pokémon or a special kind of food that distracts Pikachu from protecting Ash. Also, Pikachu might have received some false information. For example, it might have been tricked into thinking that Ash has a better chance of escaping the Spearow without it. In the heat of the moment, Pikachu makes a hasty decision based on this wrong information.
Well, the story could develop in many ways. Ash might go into a deep depression at first. He has been with Pikachu for so long that its absence would be a huge blow. He could start doubting his abilities as a Pokémon trainer.
Another reason might be related to Pikachu's own growth. As Pikachu becomes more powerful and self - aware, it might feel that it needs to go on its own journey without Ash. It could be seeking new challenges and experiences that it believes Ash can't provide, like exploring regions that Ash has no intention of going to or training with Pokémon of a different caliber.
Ash would be extremely devastated. Pikachu is not just his Pokémon but also his closest friend. He would likely first try to reason with Pikachu, not believing that Pikachu could truly betray him. He might be in denial for a while.
Ash would be completely devastated. Pikachu has been his closest companion since the start of his journey. He would probably go into a state of shock at first, unable to believe that Pikachu was gone.
Ash would be heartbroken. He would probably try to talk to Pikachu first, asking it what's wrong and trying to calm it down. He might be in denial at first, thinking it's some kind of joke or that Pikachu is just playing too roughly.
They could start by sharing their favorite things, like Ash sharing his Pokemon training experiences and the female Pikachu sharing her favorite places to play in the wild.
Brock could be a great addition. He could provide some comic relief and also be the voice of reason. For example, when Ash and Pikachu get into some crazy situations, Brock could be the one to offer practical solutions.
One popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.
Well, if Ash evolved Pikachu in fanfiction, it might change the way their relationship is perceived. Maybe Pikachu would become more independent or they'd face new challenges because of the evolution.