Some characters might be amused. For example, Sans would probably make some witty remarks about the fanfiction's plot twists. Papyrus, on the other hand, might be a bit confused at first but then get really excited if he finds a heroic role for himself in the fanfiction.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.