Definitely the Giants. They have a history of causing trouble for the demigods. Annabeth's disappearance could be part of their plan to weaken the demigod forces. They might think that without Annabeth, the demigods would be less organized in their battles.
Well, one possibility is that the other characters would be in a state of panic. Grover might use his nature - related powers to try and track her down. The demigods would likely form search parties and head to all the places Annabeth was known to frequent, like the Camp Half - Blood or the various locations in the mortal world where she had quests. They would also probably start to suspect some of the old enemies, like the Titans or the Giants, of being behind her disappearance.
Voldemort's remaining followers could be suspects. Even though Voldemort is gone, there might be some die - hard supporters who still hold a grudge against Harry. They might have kidnapped him to try and revive the dark side. Another suspect could be some jealous wizard who has an eye on Harry's wife. He might think that with Harry out of the picture, he can get closer to her.
In this fanfiction, it might be that Percy goes missing because he is on a self - imposed mission. He could be trying to protect his friends from a danger he discovers, and he doesn't want to involve them, so he disappears without a trace. He might be in a far - off land, facing challenges all alone.
Percy might be a source of comfort for Annabeth. School can be a tough place sometimes, with all the academic pressure and social drama. Percy is someone she can always rely on, and without him, she feels less secure. Maybe he has a unique way of making her laugh or feel better when she's down, and she misses that a great deal.
There could be all sorts of fanfictions. Maybe it's a story of her frantic search or her dealing with her emotions during his absence.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.
A new character introduced in the fanfiction might be the main suspect. This character could have some ulterior motives that are not clear at first. They might seem friendly but are actually behind Percy's disappearance. For example, they could be working for an evil organization that wants to use Percy for some dark purpose. So they plan his disappearance carefully to achieve their goals.
Annabeth would be in a tough situation. On one hand, she has to take care of herself and the baby. On the other hand, she's desperate to find Percy. She may be feeling a mix of fear, anger, and determination. She could be using her intelligence to analyze any clues that might lead to Percy's whereabouts. She might also be reaching out to any allies she has in the mythological world, asking for their help in finding Percy.
One possibility is that Percy clashes with the school's jocks or popular kids. Since he's an outsider, they might pick on him at first. But then Percy shows them what he's made of, not with brute strength necessarily but with his quick thinking and his ability to outwit them. He could also get involved in a school competition, using his unique skills to help his team win.
One reason could be that he was framed for a crime he didn't commit and had to run away to clear his name. In the complex world of the fanfiction, there could be a conspiracy against him. Some powerful beings might not want him to be with Annabeth or have a child, so they set him up and made him disappear.