There's a possibility that it could be a character who has been misinformed about Annabeth. They might have heard false rumors about her and in a fit of anger, punch her. Or it could be a character who is on a mission to take revenge for something they think Annabeth's group did, and she becomes the target as a representative of that group.
Maybe an antagonist who wants to disrupt the group dynamic. If Annabeth is a key part of a group, punching her could cause chaos and confusion among her friends. It could also be someone who has been influenced by dark magic or some sort of mind - control in the fanfiction. This would make them act against their normal nature and punch Annabeth without really having a personal grudge.
One possible scenario could be that it's a case of mistaken identity. Maybe someone thought Annabeth had done something wrong when she hadn't. Another could be in the heat of an argument where emotions got out of control and someone lashed out at her. Or perhaps it's part of a more complex plot where an enemy is trying to send a message to her friends by punching her.
Percy might be one. They have a close relationship, and in some fanfictions, he could be more forward with his feelings through flirting.
Well, it depends on who is flirting with her in the fanfiction. If it's someone she doesn't like, she'll probably be dismissive and give cold responses. However, if it's someone she has a bit of an interest in, she could be coy and a bit shy while still maintaining her confident facade.
Her family could be a great help. If they are aware of the situation, they can step in. Maybe her parents can talk to the school authorities or even directly to the bullies' parents. Family support is always crucial in such situations.
It could be an enemy from the underworld. Maybe a powerful demon or a vengeful spirit that has a grudge against her. They might use their dark powers to harm her, like sending a bolt of dark energy her way.
Maybe it's a human who has somehow discovered the world of demigods and is afraid of their existence. They shoot Annabeth out of fear and misunderstanding. Another option is that it's a rogue demigod who has gone bad and is working against their own kind. This character could have a personal grudge against Annabeth specifically, perhaps because of her intelligence and leadership skills which they envy.
One example could be a situation where Annabeth sees Percy getting a lot of attention from a new girl in their group. She might start acting cold towards Percy, which is a classic sign of jealousy in fanfiction. In such a story, the author might explore her insecurities about her relationship with Percy.
Percy would probably be an important character. He's Annabeth's close friend and love interest. His reaction to Annabeth getting her period could be interesting, like he tries to be supportive but is a bit clumsy at first.
Well, perhaps Rachel has some unique talents or abilities that catch Percy's attention. In the fanfiction, this might make Annabeth feel insecure about her own relationship with Percy. For example, if Rachel can communicate with the gods in a way that Annabeth can't, and Percy is impressed by it, Annabeth could get jealous.
Annabeth would be very shaken up. After the initial shock, she would assess the situation calmly. If Percy is unconscious, she would try to find a way to wake him up. She might talk to him, recalling their memories together, hoping it would reach him on some level. And she would be very protective of him while he's injured, not allowing any further harm to come to him. She would also feel guilty, thinking that if she were stronger or more careful, Percy wouldn't have had to protect her and get hurt in the first place.