It could be a story where Percy takes his shirt off during a training session. Maybe he's getting ready for a really intense sparring match and removing his shirt is part of his pre - fight ritual to show his determination and focus.
Annabeth could be one of the main characters interacting with him. She might tease him about taking his shirt off, or she could be concerned if he's taking it off because of an injury. Their relationship is complex, and this situation could add a new layer to it.
One possible scenario could be that Percy got injured and Annabeth needs to tend to his wounds on his torso, so she takes his shirt off. For example, he might have been scratched by a monster during a battle, and she has to clean and bandage the wound quickly.
Sakura might blush a bit and scold him for being so carefree. She's always been a bit more proper, so Naruto's shirt - off moment could catch her off - guard.
A reason could be that he's overheating. Danny's powers might sometimes cause his body temperature to rise, and taking off his shirt helps him cool down. It's a simple practical reason that could be used in a fanfiction to show his more human side, dealing with the physical discomforts that come with his ghostly powers.
In the context of a fanfic, there could be a bet or a dare involved. Percy lost a bet or was dared to let Annabeth take his shirt off. This could lead to some humorous or interesting interactions between them as they both deal with the situation.
One possible reason could be that she suspects Percy has some sort of mark or injury on his torso. Maybe they were in a recent battle or encounter and she wants to check if he's okay.
In the fanfiction, Robin might reveal a secret identity or a hidden face with a unique scar or mark. It could also lead to a moment of great vulnerability, where his friends see a side of him they never expected.