Annabeth would likely be furious at first. She has been through so much with Percy, and for him to leave her for Calypso would seem like a betrayal. But then, she might also start to reflect on their relationship and what went wrong. She could become determined to win Percy back or to move on and prove that she doesn't need him.
One reason could be that she's scared. The idea of becoming a mother at that point in her life might be overwhelming, and she might think she can't handle it with Percy. Maybe she wants to protect Percy from the difficulties that come with the pregnancy as well.
Well, Percy is a complex character. His reaction could depend on how much he knew about the pregnancy before Annabeth left. If it was a complete surprise, he might be in denial for a while. He could also start to blame himself, thinking he had somehow failed her. And then he might eventually turn that pain into a determination to be a better person, whether or not he ever sees Annabeth again.
One popular 'Annabeth leaves Percy' fanfic might be 'Fractured Bonds'. It explores how Percy copes with her departure and the changes it brings to his life. Another could be 'Alone Again' which delves deep into the emotional turmoil Percy experiences after she's gone.
Annabeth would probably be very hurt at first. She has a strong bond with Percy, and his leaving for someone else would be a huge blow to her self - esteem and her heart.
Well, Percy could have a range of reactions. At first, he might be in denial, not believing that Annabeth would actually leave him for a job. Then, as it sinks in, he could get angry. He's a passionate guy, and this would be a big blow to him. He might also feel a bit of envy towards the job that seems to have taken Annabeth away from him. In the long run, though, he could use this as a chance to grow on his own and become more independent.
Well, perhaps it could start with Percy feeling unfulfilled in his relationship with Annabeth. He might meet Calypso and be enchanted by her. Calypso could offer him a different kind of life, away from the battles and stress with Annabeth. Maybe Percy starts to see Annabeth more as a comrade than a lover at this point and decides to leave her for Calypso.
Annabeth is known for her intelligence and strategic thinking. She is a strong and independent character. Percy, on the other hand, is brave and has a strong connection to the water due to his father being Poseidon. In the fanfic, when Annabeth sits in Percy's lap, it might be a way to show their unique relationship which has developed over the course of their many shared experiences in the Percy Jackson universe.
Perhaps Annabeth is forced to leave because of a prophecy. She believes that by leaving Percy with the child, she is protecting them both in some way. Percy then has to struggle with his own feelings of abandonment while also trying to raise the child. He might seek help from his friends at Camp Half - Blood, like Grover. Grover could help him understand the child's needs better as he has some experience with young creatures. The story could then follow Percy's journey of self - discovery as he becomes a more responsible and loving father.
Perhaps there are external factors at play. The world of demigods is full of chaos and danger. Reyna might be in a position where Percy feels he has to be by her side to protect her or to fulfill some greater prophecy that involves the two of them, and this leads to him leaving Annabeth.