Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
The Beast would likely be extremely worried and angry at himself. He might immediately rush to her side, blaming his own cursed existence for putting her in danger. His first instinct would be to protect her and find a way to ease her pain.
Another reaction could be that he tries to hide his feelings by putting on a brave face. He might joke around even more to cover up the fact that he's really hurt. But deep down, his feelings are still damaged and this could lead to some bottled - up emotions that might explode later.
The Beast would be very confused at first. He has been the one with the beastly form for so long, and now seeing Belle in that state, he might feel a sense of guilt as if it was his fault. He would be protective of her, though, and try to help her adjust. Gaston, on the other hand, would probably see this as an opportunity to prove his 'heroism' by trying to 'save' Belle from her new form, but really he just wants to show off. Belle's father would be heartbroken and try to find a cure for his daughter, using his knowledge of inventions and books.
Dazai is not based on a true story. Usually, characters like Dazai are formed through the author's imagination and artistic expression to create an engaging narrative for the readers.