Serena could be in shock at first, but then she would probably show her appreciation to Ash. She might say some heartfelt words to him, like how she knew she could count on him. And then she might stay close to Ash for a while, feeling safe in his presence.
At first, Serena may be a bit dazed from the whole experience. But as she realizes that Ash has saved her, she would break into a big smile. She would likely hold Ash's hand tightly, not wanting to let go. She might start talking about how scared she was, but how his presence made her feel better. She could also be inspired by Ash's bravery and want to become stronger herself, so that she can also be helpful in future situations. She might even offer to cook Ash his favorite meal as a thank - you for saving her.
She might be very grateful and give Ash a big hug.
In this fanfiction, Ash could first assess the situation carefully. He might notice that Team Rocket has set up some sort of elaborate scheme to capture Serena. So, he starts by sending out his Pokémon like Bulbasaur to use Vine Whip to disarm any mechanical devices Team Rocket has. Then, he could bravely run towards Serena while his Pokémon keep Team Rocket at bay. He might also encourage Serena to be brave and follow his lead as they make their escape. Once they are out of immediate danger, he could make sure Serena is okay and comfort her after the scary encounter with Team Rocket.
Serena could be a bit shy at first. She might blush deeply and look down for a moment. But then, she would look into Ash's eyes with love and admiration. She would tell Ash how much she loves him and how she has been hoping for this moment. She would accept his proposal gracefully, and they would share a tender embrace while their Pokémon friends cheer around them.
They would probably be a bit shy at first. Ash being his usual energetic self but also respectful of Serena's space in the tent. Serena might be a bit nervous but also excited to be sharing such a close space with Ash.
Their Pokémon might be very protective. For example, Pikachu could stay closer to Serena all the time, sensing the new life. Some Pokémon might bring small items like berries or soft leaves, thinking it could be useful for Serena or the baby.
Pikachu would probably be very excited. It might jump around Ash, making little electric sparks of joy. Bulbasaur could bring Serena some fresh berries it finds in the forest as a sign of care. Eevee, if they have one, might curl up next to Serena's belly, as if protecting the unborn baby. Their Pokémon would sense the importance of this new development and do their best to be helpful and protective.
Some of their Pokémon friends might also show unique reactions. Pikachu could be very protective of Serena, always staying close by to make sure she's okay. And their other Pokémon might bring little gifts like berries or shiny stones they find, as if they understand what's going on. The more mischievous Pokémon might play little pranks that end up being endearing in the context of the pregnancy, like stealing Serena's hat and then replacing it with a flower crown as a sign of celebration.
In many Pokemon fanfictions, Ash might comfort Serena by simply being there for her. For example, if Serena is feeling down about a failed performance in a Pokemon Contest, Ash could offer words of encouragement like 'You'll do better next time, Serena. I've seen how hard you practice.' He might also share his own experiences of facing setbacks in Pokemon battles to make her feel that she's not alone in facing difficulties.
The fanfiction could start with Dawn being in a vulnerable situation, perhaps alone in an area where Team Rocket has set a trap. Ash, who has a sense of justice and cares about Dawn, discovers the situation. He then uses his knowledge of Pokémon battling, combined with his quick thinking, to outwit Team Rocket. His Pokémon work together as a team, and through a series of exciting battles and maneuvers, he manages to rescue Dawn from the clutches of Team Rocket.
In fanfiction, anything can happen. However, without specific details of the particular fanfic, it's hard to say for sure if Serena is pregnant. Some fanfictions might explore this plotline for the sake of drama or character development. It could add an interesting twist to the relationship between Ash, Serena and Lemon, but it also depends on the overall theme and direction the author wants to take the story.
Maybe Ash is portrayed in a way he doesn't like in Serena fanfiction. For example, the stories might change his character traits or put him in situations he wouldn't be in the original series.