Well, Percy is used to strange things, but the multiverse would still throw him for a loop. In the fanfiction, he could start off being skeptical. After all, he's been through a lot in his own world, and the idea of countless other versions of his reality seems far - fetched at first. But as he delves deeper, he might start to see parallels between his world and the other universes in the multiverse, which could make him more accepting.
Obviously Percy himself is very popular. Since it's a 'Percy Jackson Reacts' scenario, he's at the center of it all. Fans love to write about his reactions.
Another interesting scenario could be Percy reacting to fanfics that change the outcome of major battles in the original series. He might be shocked or proud to see how fans rewrite history, like if a fanfic has him winning a battle in a completely different way than he actually did. This can show different perspectives on his powers and leadership skills.
Percy Jackson has a curious personality. In the context of the multiverse fanfiction, this curiosity will lead him to want to understand how the multiverse works. He'll be asking questions and trying to figure out the rules. His brave and head - strong nature means he won't shy away from any threats or challenges in the multiverse. He'll face them head - on, whether it's a menacing version of a monster from his world or a completely new threat from an unfamiliar universe. His personality traits combined will shape his reactions in the fanfiction in a very dynamic way.