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.
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.
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.
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.
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.
One common theme could be her feeling of isolation. In the Warrior Cats series, Squirrelflight has had many complex relationships and situations. In fanfictions about her depression, she might be isolated from her Clan due to misunderstandings or past mistakes. Another theme could be her struggle with self - identity. She has had issues with her family and her role in the Clan, and this could be exacerbated in a depressed state, making her question who she really is and what her purpose is.
One popular 'naruto gets depressed fanfiction' is 'The Depths of Naruto's Despair' where Naruto faces a series of setbacks that lead him into a deep depression. It shows his struggle to find hope again.