Thalia might be indifferent. She could just nod and say something like 'That's cool' without really getting into it. Because she may not be as interested in fanfiction as Annabeth is, especially if it's about Percy. But she still respects Annabeth's interest.
She might be a bit surprised at first. After all, fanfiction isn't always something they talk about. But then she could be interested and ask Annabeth more about it, like what kind of stories are in the fanfiction.
Maybe they were just having a casual conversation about their interests in different fictional works related to their group. Since Percy is an important character in their story, Annabeth might have discovered some really well - written Percy fanfiction that she couldn't help but share her liking for with Thalia.
Well, Percy might be initially shocked. He could stand there for a moment, his mind racing with a million thoughts all at once. Maybe he would then break into a huge grin, excited about starting a family with Annabeth. After all, they've been through so much together, and this could be seen as a new adventure for them.
She might be shocked and angry. Thalia is a protective character. She could immediately vow to find those who abused Percy and make them pay. For example, she might start by gathering information about the abuse, asking Percy for details in a gentle way to not further traumatize him.
She might be shocked at first. Then she could become extremely protective of Percy. She would likely confront those who are responsible for the abuse and make sure Percy is safe from further harm.
She's probably super excited. She might run and give him a big hug right away.
Annabeth could be shocked at first. She might not have expected Percy to be so straightforward with his feelings. She would need a moment to process what he's said, and then she might respond in a more measured way, like asking him some questions to make sure he's serious.
Annabeth's reaction could vary. If she's been missing Percy a great deal, she'll be overjoyed. She might show him around the school, proud to introduce him to her friends or teachers (if possible in the context of the fanfiction). However, if she's in the middle of something important like a test or a big project, she could be a bit annoyed at first but still be happy to see him deep down.
She could also become cold and distant. Instead of engaging in a shouting match, she might simply walk away and give Percy the silent treatment. This can be a powerful reaction as it makes Percy realize that his yelling has pushed her away and he has to make an effort to fix things.
She might be initially very angry. Annabeth is intelligent and perceptive, so she would likely feel betrayed that Percy thought he could deceive her. For example, if Percy lies about a mission, she could find out the truth later and confront him immediately, demanding an explanation.
She might be shocked at first. Since they are usually comrades or friends, Percy screaming at her is out of the ordinary, so she could just stand there in disbelief for a moment.