Alphys would likely be very confused at first. She's used to her own types of fan - related interests, and this would be something new and strange. She might think it was some sort of strange new trend in the Undertale fandom that she didn't know about. As she thought more about it, she could become more accepting, especially if she realized it was just a creative expression of the fans. But it would definitely take her some time to get used to the idea.
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.