Annabeth would probably be determined to get Percy's memory back. She could start by researching ways to reverse memory loss in the world of their mythology. She'd likely be angry at whatever or whoever caused Percy to forget her and would be on a mission to fix it. Maybe she'd also be a bit scared that she might lose Percy forever if she can't make him remember.
She might be heartbroken at first. She could try to jog Percy's memory by showing him things that were important to them.
Annabeth would initially be in shock. She has such a deep connection with Percy, and for him to forget her would be like a part of her world collapsing. She would then turn to her wits and knowledge. Since she is so intelligent, she'd start looking for clues in their past, like old battle sites or gifts they exchanged. She might also enlist the help of their friends. For example, she could ask Grover to use his nature - related powers to find some sort of memory - restoring herb or something. All the while, she would be dealing with her own emotions of sadness, frustration, and love for Percy.
Annabeth could also be angry, not at Percy but at the situation or the forces that led to his death. She might vow revenge and start on a journey to find out who or what was responsible for Percy's sacrifice and make them pay. This could lead her into a whole new set of adventures and challenges as she tries to come to terms with her loss.
She'd probably be shocked at first. But then her face would light up with joy. After all, she and Percy have been through so much together.
Percy might be a bit flustered and embarrassed. He's protective of his relationship with Annabeth, so having Magnus walk in might make him feel like their privacy has been invaded. He could quickly try to shield Annabeth or just look really shocked.
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.
Then, she would probably be angry. Annabeth is a strong - willed character. She would feel betrayed and used, and her anger would fuel her to find out the truth. She might start her own investigation to figure out what's really going on with Percy, whether he's been influenced by someone or if there's another reason for his actions.
Maybe he was hit by some magic or a spell that affected his memory. It could also be due to some sort of trauma he experienced that made his mind block out his memories of Annabeth.
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.
She could be heartbroken. Annabeth has been through a lot with Percy, and to be rejected for Rachel might make her feel like all their adventures together meant nothing.
Annabeth could be shocked at first. She might not have expected Percy to be so straightforward with his feelings. She would need a moment to process what he's said, and then she might respond in a more measured way, like asking him some questions to make sure he's serious.
On the other hand, she could try to honor Percy's sacrifice. Annabeth might focus on the reasons why Percy died for her and strive to fulfill any tasks or goals that he might have hoped she would achieve. She would carry on his legacy in a positive way, even though she is heartbroken.