Since we don't know the details of the story, it could be any woman. It might be a fictional character created to illustrate a point about fears and phobias.
I'm not sure specifically as I haven't heard this exact story. It could be about a woman who has an irrational fear of olives, perhaps due to a past experience like getting sick after eating one, or maybe it's a cultural or family - related fear that has been instilled in her.
It could be that the balloon popped suddenly and gave him a fright. Or perhaps there was a strange noise or shadow that made him scared while holding the balloon.
Maybe it's because Percy in the stories often challenges the traditional Roman beliefs and ways. His actions and abilities might seem threatening to the Romans in the fanfiction world.