Percy would likely be very confused at first. He'd probably feel a mix of anger and sadness. He might start to question his own actions and wonder if he did something wrong to make his sister leave.
Reacting to Percy Jackson fanfiction is quite an adventure. One aspect that can be really engaging is when the fanfiction delves deeper into the backstories of the minor characters. It gives them more depth and makes the whole universe feel more alive. Also, some fanfictions take a darker turn, exploring the consequences of the battles and the traumas the characters might have endured. It can be a bit different from the original series, but that's the beauty of fanfiction.
Percy might be initially very confused and hurt. He may try to find his sister and understand why she has left him. He could also become very angry and feel betrayed, which might lead him to take some rash actions.
One possibility is that the sister has some unique powers that the Olympians fear. So they abandon her. In the fanfiction, she could be on a journey to discover her true self and her powers, while Percy unknowingly gets involved in her quest. Maybe she's being hunted by some mythical creatures because of her heritage, and Percy has to save her in the end.
One possible plot could be Percy feeling a deep sense of betrayal and loneliness. He might struggle with his self - worth as he wonders why his sister left him. This could lead him on a journey to find answers about his family and his own identity, perhaps even uncovering secrets about his demigod heritage that his sister knew and he didn't.
It could be a vengeful god. Maybe a god who feels that the sister's existence is an insult to their power. So they set out to stop Percy from finding her. For example, Hades might be the antagonist if he believes that the sister's return could disrupt the order in the Underworld.
If it's a well - crafted fanfiction, it might start with an attention - grabbing opening. For instance, if it begins with Percy in a situation that seems impossible to escape from, it immediately hooks the reader. I like it when the fanfiction respects the lore of the original series while still being creative. Maybe it has Percy using his powers in a new and unexpected way.
She might be shocked at some of the wild ideas in the fanfiction. Maybe she'd laugh at some of the more outlandish plot twists.
Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
Annabeth would be completely devastated. She might go into a state of shock at first, not believing that Percy is really gone. Then she would be filled with anger and a determination to find a way to bring him back, if possible. She could start researching ancient magic or prophecies that might reverse his death.
One idea could be Percy feeling betrayed by Poseidon and then finding a new family among other outcasts in the demigod world. He could form a group with them and go on adventures that are not related to Poseidon at all.