Percy might initially feel a great deal of anger. He's been wronged by those he thought were his friends. But then, with Annabeth by his side, he could start to focus on finding out the truth. He might use his skills, like his combat abilities and strategic thinking, to figure out who or what is behind the betrayal.
One possible reason could be jealousy. Maybe Percy has some special abilities or achievements that make others envious. For example, his connection to the sea could be seen as a huge advantage that others might resent. Another reason could be misinformation. Someone might spread false rumors about Percy, leading others to believe he's done something wrong when he hasn't. It could also be that there's a misunderstanding about his relationship with Annabeth. Some might think he's getting special treatment from her or the other way around, which causes them to turn against him.
In some fanfictions, Annabeth might betray Percy for power. Maybe she gets lured by some dark force offering her great power and in order to get it, she has to turn on Percy. It could also be due to a misunderstanding. For example, she might be tricked into believing Percy has done something really bad and as a result, she betrays him without realizing the truth.
At first, Percy would likely be angry. He's a fighter, and being betrayed by someone he loves would fuel his rage. He might go after Annabeth to demand an explanation. But as the reality sinks in, he could become very sad and withdrawn. He might isolate himself from his other friends as well, feeling like he can't trust anyone anymore. He would constantly be thinking about what he did wrong to make Annabeth turn on him.
Perhaps Percy's loved ones are made to believe false rumors about him spread by an unknown enemy. As a result, they turn their backs on him. Percy would need to find a way to expose the real culprit behind the rumors and regain the trust of his friends and family.
She might focus on her studies more. Since Annabeth is really into knowledge, she could pour all her energy into her schoolwork, like spending extra hours in the library or on research projects.
Another reason could be jealousy. Suppose Percy was getting a lot of attention and praise for his recent achievements, and Annabeth felt overshadowed. Her jealousy could fester and turn into a desire to undermine Percy, leading to her betrayal. However, this would be a more complex and less noble reason compared to the others.
One possible reason for Annabeth to betray Percy in fanfiction could be a misunderstanding. Maybe she was misled by false information about Percy's actions or intentions. For example, if someone spread rumors that Percy was secretly working against the demigods, Annabeth might believe it without getting the full story.
Percy would likely feel a great sense of hurt and confusion. He might question his own judgment in trusting Annabeth. He could become more withdrawn and less trusting of others in general.
Well, Annabeth is a strong character. She may start by grieving privately, not wanting to let her vulnerability show. As she is pregnant, she would feel an even greater sense of responsibility. She could start documenting stories about Percy for the baby, so that the child can know who their father was. And she would probably try to find a new purpose in life, centered around raising the baby in a way that Percy would have wanted.
In the fanfic, there could be a big revelation later on that it was all a misunderstanding. Maybe some sort of mind - control magic was involved. But until then, Percy would have to face many challenges alone. He might even start to question his own worth and abilities. For example, he could be hesitant to use his powers, thinking that if his friends could betray him, maybe his powers are part of what made them turn away. It would be a very emotional and angsty journey for Percy as he tries to figure out what went wrong and how to fix it.