Another option could be someone from a different mythology or realm. For example, an elf or a demigod from another pantheon. This would add an interesting twist to the fanfiction as it would bring in new elements and interactions between different mythologies and the mortals.
The fanfiction 'Mortals meet Annabeth's boyfriend' might have the mortals being really curious about their relationship. They could start asking all kinds of questions about how they met, what they like about each other. And perhaps Annabeth's boyfriend has to prove himself to the mortals in some way, like showing his strength or intelligence to gain their respect.
Typically, there are curious students. They are often in a learning environment, like a high school or college, and they accidentally stumble upon Percy and Annabeth's secret. They might be studying mythology and are really interested in finding out if the stories are real.
Well, they might have to hide their demigod powers. For example, Percy could be really tempted to use his water - controlling abilities when they are near a big fountain in front of mortals, but he has to hold back. Annabeth would be using her wisdom to come up with quick lies if the mortals start asking too many questions about their strange behavior or the weapons they carry.
In some fanfictions, mortals might meet Percy and Annabeth in a coffee shop. Percy could accidentally use his powers to make a small water fountain out of a cup, and Annabeth would quickly cover it up while the mortals are left wondering what just happened.
Grover could be one of Percy's friends to meet Annabeth. He's a satyr and has been with Percy through a lot. When he meets Annabeth, he might be really happy to see another important person in Percy's life.
In many fanfictions, when Annabeth's friends meet Percy, there could be a mix of reactions. Some might be initially cautious, especially if they've heard of Percy's dangerous adventures. But often, they end up being impressed by his skills and his loyalty to Annabeth.
It could be for the greater good as she perceives it. Annabeth might think that by betraying Percy, she can save more people or prevent a greater disaster. However, her perception might be wrong, and she realizes it too late after the betrayal has already occurred.
They interact cautiously. Since they are demigods and their world is full of secrets, they don't want to expose themselves. Percy might be a bit more laid - back in his interactions, cracking jokes with mortals while still being vigilant. Annabeth, on the other hand, is more calculating. She'll study the mortals first to see if they can be trusted.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.
It could become more professional. They might keep their personal feelings aside and just focus on getting the job done.