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.
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.
Serena could be a fictional story crafted by the author's imagination. But sometimes, fictional works can touch on universal themes and emotions that feel true to many people's experiences.
Her partner would be a very likely character to be involved. They could be the source of support, or in some cases, the source of conflict if they have different views on the pregnancy. Her best friends are also likely to be in the fanfic. They might be there to throw baby showers, give advice, or just be a shoulder to cry on.
There are several characters that would work well. Lillie from the Alola region could be a great addition. She has her own unique story and relationship with Pokemon that could contrast with Serena's. Another option is Kiawe. His fiery personality could create some interesting dynamics with Serena. Then there's Gladion, who could bring an element of mystery and a different approach to Pokemon battles. And of course, Professor Oak. His wisdom and knowledge of Pokemon could be a valuable asset in a Serena Pokemon fanfic. He could be the one who sends Serena on a special mission or gives her some crucial information about a rare Pokemon.
Ash could be the cause. He might accidentally break a promise to Serena, like promising to watch her perform in a Pokemon Showcase but then missing it because he got caught up in a Pokemon battle. This would surely make Serena angry.
Another possible main character could be a rival trainer. Maybe someone who has been in the Pokemon world for a long time and initially looks down on Serena. But as Serena progresses, they develop a mutual respect. Also, a Pokemon professor, like Professor Oak, could be in the story. He could provide Serena with valuable knowledge about Pokemon, such as how to catch and train them.