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.
Percy might be really shy and hesitant at first. He could blush and stutter, not expecting such a request from Annabeth.
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.
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.
Annabeth is known for her intelligence and strategic thinking. She is a strong and independent character. Percy, on the other hand, is brave and has a strong connection to the water due to his father being Poseidon. In the fanfic, when Annabeth sits in Percy's lap, it might be a way to show their unique relationship which has developed over the course of their many shared experiences in the Percy Jackson universe.
Percy might develop more of a steely determination. In Tartarus, he has to face extreme danger, which could make him even more resolute in protecting Annabeth and getting out. Annabeth, on the other hand, could become more resourceful. She'd have to use her wits in this strange and deadly place.
She might take it because she loves the smell of it. It has Percy's scent, which makes her feel close to him.
One possible place could be in the Underworld. It would be a very unexpected and dangerous place, but that would add to the drama. They might be there on a quest and find a moment of peace and passion among the chaos of the dead.
Annabeth might be initially speechless. She'd look at Percy with a mix of disbelief and love. After a moment, she would throw her arms around him and say 'yes' while laughing. She would then start thinking about all the adventures they could have as a married couple and how they could build a future together in both the mortal and demigod worlds.
In this fanfic, Percy could develop a darker side. The horrors of Tartarus might make him more ruthless when it comes to protecting himself and Annabeth. Annabeth, on the other hand, may become more empathetic. She could see the pain and suffering of the other beings trapped in Tartarus and try to help them, which would shape her character in a new way. Also, their relationship would develop further. They would have to trust each other completely, and this could lead to a more mature and deep connection between them.
She might initially be shocked. Annabeth is a strong character, so she could start by trying to stand up to Percy verbally, asking him why he's being so mean.