Percy could be heartbroken. He might initially go into a state of denial, not believing that Annabeth would actually leave him for a mortal. He might keep thinking there was some mistake or that she was being influenced in some way.
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.
Well, it's possible that such a fanfiction exists given the creativity of fanfiction authors. They like to experiment with different relationship dynamics. But considering the popularity and the nature of Annabeth and Percy's relationship in the source material, this would be quite a shocker. It could also attract a lot of criticism from fans who are attached to the original relationship, while others might be interested in seeing how such a different plot would unfold.
It could also be a matter of self - discovery. Annabeth might have been on a journey to find out who she was outside of her relationship with Percy. Meeting this mortal was a turning point where she realized she wanted to explore a different path, even if it meant leaving Percy. Maybe she thought that being with a mortal would give her a chance to have a more ordinary life, at least for a while.
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.
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.
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.