It could also be that Ash wants to protect Serena from the harsh reality of battles. By hiding his scar, he is sort of shielding her from the more violent and dangerous aspects of his journey as a Pokemon trainer.
Maybe Ash doesn't want Serena to worry about him. He might think that the battle scar is a sign of his vulnerability and he wants to always appear strong in front of Serena.
She might be a bit angry at first that Ash hid it from her. But then she would probably feel sad that he felt the need to hide it and show a great deal of concern for him.
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.
Maybe she has her own vision of their relationship and the fanfiction distorts it in ways she doesn't like.
She might be very grateful and give Ash a big hug.
Well, it could be that Ash's aura has a special connection with Serena's in the fanfic world. His aura might be able to transfer some of his energy to Serena when she's weak or injured. He could focus his aura to create a protective shield around her, shielding her from attacks like from wild Pokémon or bad guys. And this way, he saves her using his aura abilities.
One possible reason could be a miscommunication. Maybe Serena did something that Ash misinterpreted. For example, she might have made a decision about their pokemon journey without consulting him, and Ash, being independent, took it the wrong way.
Maybe Ash's character in the original work has a certain fixed image, and some fanfics about Serena deviate from this image he represents, so he hates it.
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.
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.
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.