Well, in the fanfic, Bruce would probably become more withdrawn. He'd brood over Jason's death, questioning his methods and his role as a mentor. Dick would be angry at the world, especially at the villains who might have been involved. Tim, on the other hand, would be more determined to prove that he can be a worthy Robin, but also carry the pain of losing Jason. Alfred would be the one constant, trying to comfort everyone with his wisdom and care. It's a really tragic event for the Batfamily and their reactions would be complex and full of emotions.
The guild would be devastated. Natsu would probably be one of the first to be enraged. He'd vow to find whoever or whatever was responsible and make them pay. His fire would burn hotter than ever as he channels his anger.
Gray would be devastated. He might go into a state of deep depression, isolating himself from the rest of the Fairy Tail guild. He would blame himself for not being able to protect her, especially if her death was related to her love for him. Gray would probably be filled with anger as well, and he would vow to take revenge on whoever was responsible for her death.
Mr. Schuester would be in shock. He saw Rachel as one of his star pupils and a driving force in the Glee club. He would struggle with the guilt of not being able to protect her. He would try to keep the club going as a way to honor her memory, but it would be very difficult for him. He might also start a scholarship fund in Rachel's name for future Glee club members.
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.
Twilight Sparkle would probably be devastated. She looked up to Celestia as a mentor, so she might go into a state of deep mourning at first. But then, she could be motivated to try and fill the void Celestia left, using all the knowledge Celestia had imparted to her.
Harry Potter might react with a newfound understanding. In the original story, he named his son after Snape. In fanfiction, this could be explored more deeply. He might feel guilty for not seeing Snape's true intentions sooner. Hermione could react with her typical intelligence - led analysis. She might try to piece together Snape's actions over the years and come to a more complete understanding of his character. Ron, on the other hand, might be more straightforward in his grief. He might be sad that someone who was on their side all along had to die in such a way.
Hogwarts as a whole might have a complex reaction. The students could be divided in their feelings. Some might be outraged, feeling that it goes against the essence of the story they love. The professors, on the other hand, could see it as an opportunity to teach about the power of imagination and the fluidity of storytelling. They might encourage students to write their own responses to these fanfictions, either in agreement or in opposition, to promote critical thinking skills.
In many fanfics, the Bat Family is often devastated. Bruce might be filled with guilt, blaming himself for not being able to protect Jason. Dick could be angry and determined to find out who was responsible. Tim might feel a sense of loss and also a strange connection to Jason, as he took on the Robin mantle after him.