It could be a new character, like a powerful wizard who has a grudge against Percy for some reason. Maybe Percy accidentally disrupted one of his spells, and the wizard is out to get him, which makes Percy mad.
One possible plot could be that Percy gets mad because his friends don't believe him about a new threat he has discovered. He might storm off and try to face the threat alone, leading to all sorts of dangerous situations.
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.
It can strain their relationship. When Annabeth is mad, there will be tension between them. They might not communicate as effectively, which could lead to misunderstandings. For example, if Percy doesn't understand why Annabeth is mad, he might say or do the wrong things, making the situation worse.
Well, in many fanfictions, Percy might be seen as too ambitious and career - focused. He could have made choices that put his job at the Ministry above his family. For example, he might have supported Ministry policies that the rest of the Weasleys disagreed with, like their stance on Harry Potter or how they were handling the growing threat of Voldemort. This could lead to the Weasleys being mad at him for seemingly choosing the wrong side or not standing by the family values.
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.
Well, in some fanfictions, Percy might be mad at Annabeth because of a misunderstanding. Maybe she made a decision without consulting him in a crucial situation regarding their adventures. For example, if she chose a path that put their friends in danger without his input, Percy could get angry.
Since I don't have access to the 'percy is mad fanfiction', it's difficult to give a detailed account. However, generally speaking, if Percy is mad, the fanfiction could be exploring his emotions deeply. It might show how his anger affects his actions, decisions, and relationships. For example, if it's a fanfiction related to Percy Jackson, his anger could lead him to take risks he wouldn't normally take, or it could cause rifts in his friendships with Annabeth or Grover. It could also be a story about how he overcomes his anger and grows as a character.
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.
Another possibility is a jealous rival. Maybe there's someone who is jealous of Gohan's power or his relationship with certain characters. This rival could do things to annoy Gohan and push him to the edge of his temper.