The setting might be a common element. If the original Percy stories are set in a particular world, the fanfictions she likes might expand on that world, adding new locations or cultures. For example, exploring more of the underwater world where Percy has some connections.
There's a possibility that Erika is part of a community that loves Percy fanfiction. Through this community, she has been exposed to various works and has gradually developed a love for them. Also, the themes in Percy fanfiction, such as friendship, heroism and magic, might resonate with her on a personal level.
It can be considered normal. People often get interested in fanfiction for various reasons. In this case, Percy's brother might like Annabeth fanfiction because he admires her character traits in the original story. The fanfiction could offer new perspectives on her, or it might be that he simply enjoys the writing style of the particular fanfictions he reads.
It could mean that Percy, as a character or a person named Percy, has an interest in the fictional stories created by fans about Annabeth. Maybe he enjoys the different interpretations of Annabeth's character, relationships, or adventures in these fan - made stories.
Well, in some versions, Percy might be initially shocked. He could be like 'Whoa, I had no idea!'. Then he might start to see Nico in a new light, with more understanding and compassion.
One possibility is that it reveals her creative side. Fanfiction can be very creative, and by liking it, she may be drawn to the unique ideas and storylines that fans come up with. It could also suggest that she's a bit of a fangirl herself, not just in the context of the original story but also in the world of fan - created content about her and Percy.
One reason could be that he enjoys the creativity of the fanfiction writers. They might come up with interesting storylines involving Annabeth that he finds really engaging. For example, they could create new adventures for her or explore her relationships in more depth. Also, if he likes the world of Percy and Annabeth, the fanfiction gives him more of that world to explore. He can immerse himself in different scenarios with Annabeth as the central figure, which is exciting for him.
Well, 'Piper's Secret Admiration for Percy' is also a great one. In this fanfiction, it might focus on Piper's internal thoughts about Percy, like how she might be secretly rooting for him during his quests and how her like for him might affect her own actions and decisions.
Archive of Our Own (AO3) is also a great place to look. It has a very organized tagging system. Just search for tags related to Piper and Percy, and you should be able to find the relevant fanfictions.
I'm not entirely sure who Erika and Klaus Mann are specifically in this story without more research. But they are likely the main characters. They could be siblings or just two individuals bound by a common experience in the context of this 'escape to life' story.