In a fanfic, there could be a new powerful enemy who targets Annabeth specifically. Percy gets mad that someone would dare to go after her. He immediately jumps into action, using his water powers to create a barrier around her and then engaging the enemy in a fierce battle, showing no mercy as he is so angry about the threat to Annabeth.
Maybe in the fanfic, there's a group of mortals who accidentally stumble upon the demigod world and start treating Annabeth disrespectfully because she's a girl. Percy, being mad at their unfair and chauvinistic attitude, uses his skills and his status as a son of Poseidon to protect her. He makes it clear that she is not to be messed with and that he'll always be there to defend her.
One common scenario is when Annabeth's over - intelligence leads her to overrule Percy's ideas in public. Percy, being a proud character, might get mad as he feels undermined. For instance, in a battle strategy discussion, if Annabeth dismisses Percy's plan without proper consideration just because she thinks her own is better.
One scenario could be that Annabeth accidentally revealed Percy's secret identity to someone he didn't want to know. This could put him in danger and make him furious with her.
One scenario could be that Percy brings in a strange and cute little magical creature that he found. Annabeth, being curious, can't help but be distracted by it. He could also start singing a really off - key song that makes Annabeth stop what she's doing and either laugh or be really annoyed, but either way, she's distracted from her original task.
Perhaps Percy forgets an important promise he made to Annabeth. In their relationship, promises are significant. If he fails to keep one, like a promise to help her with a project or be at a certain place at a certain time, Annabeth would surely be mad. It could also be that Percy accidentally breaks one of Annabeth's creations or possessions, which she holds dear.
Well, it could be that Annabeth constantly corrects Percy in front of others. For example, when Percy is making a plan to defeat a monster, Annabeth keeps interrupting and saying his plan is all wrong. This public humiliation might make Percy mad.
One common scenario is when another girl shows interest in Percy. For example, if a new demigod arrives at Camp Half - Blood and she's constantly around Percy, Annabeth might get jealous. She could start acting cold towards Percy or be overly critical of the new girl. Another situation could be if Percy has to go on a quest with a female partner and they seem to be getting along really well. Annabeth might worry that Percy will develop feelings for this other girl.
One possible scenario could be that Percy lies about a dangerous mission he's about to embark on. Maybe he doesn't want Annabeth to worry and so he tells her he's just going on a simple errand. Another could be that he lies about his feelings towards someone else in their group, like saying he doesn't trust a new member when in fact he does, just to avoid some potential conflict within the group that he thinks Annabeth would get involved in.
One possible reason could be a misunderstanding. Maybe Annabeth made a decision without consulting Percy, like choosing a quest that she knew was dangerous but didn't tell him until it was too late. Percy might feel left out and mad about it.
One popular 'Percy protects Annabeth' fanfic could be 'Percy's Vow'. In this fanfic, Percy makes a solemn promise to always keep Annabeth safe. It shows Percy using his powers and quick thinking to fend off various threats that come their way. Another might be 'Guardian Percy' where he takes on the role of Annabeth's personal protector during a dangerous quest. The author beautifully depicts Percy's determination and loyalty in safeguarding Annabeth.
A typical scenario could be when Annabeth spends a lot of time with another male character while working on a project. Percy might see them getting along really well and start to feel jealous. For instance, if they are building something together in the fanfic, like a new weapon or a structure, and Percy feels left out.