Shadow could also play a role. He has his own complex relationship with Sonic. His reaction to Sonic's death might be a mix of respect and perhaps a sense of loss of a worthy adversary. He could either help or be a source of conflict depending on how the story is written. And of course, the Chaotix detective group could be involved in investigating Sonic's death as well.
One possibility is that Amy would turn her grief into motivation. She could take on Sonic's role in protecting their world. She'd train harder, be more vigilant, and vow to defeat any threats that Sonic used to face. Her reaction could also involve her reaching out to Sonic's other friends, and together they could form a stronger bond in his memory, while also dealing with their own individual griefs.
Tails could be one of the main characters. He might play a role in helping Amy realize that her chasing Sonic isn't healthy. For instance, he could give her advice based on his own observations.
Her best friend could be a main character. Maybe someone named Sarah who is always by her side, bringing her soup and medicine.
There could be a wise old wizard - like figure in the other dimension who guides Amy. He has lived for centuries and knows all the secrets of this strange place.
Knuckles might also be part of the story. As the guardian of the Master Emerald, he has a sense of responsibility. He could either be involved in the events that led to Sonic's fall or be part of the rescue mission. His strength and knowledge of the area (if Sonic falls in a location related to the Master Emerald) could play an important role.
Obviously, Sonic and Amy are the main characters. Sonic, the speedy blue hedgehog, and Amy, the determined pink hedgehog. There might be other characters as well, but they are likely to be in supporting roles compared to Sonic and Amy.
I'm not sure about its popularity. It could be popular within a niche group of Sonic fans who are interested in more extreme or non - traditional storylines. However, it might not be as well - known to the general Sonic fanbase as it has a rather specific and somewhat disturbing concept.
Well, the 'angel of darkness' is likely to be a main character. It might be an entity with its own motives and powers. Another character could be Sonic. He might play a role in trying to save Amy Rose from the influence of the 'angel of darkness' or be involved in some way in the events that unfold.
I'm not sure specifically as I haven't read this particular fanfic. But it could potentially be a story where Amy, a character, does something that causes Sonic to have a serious injury like a broken back. It might be a very dramatic or action - filled plot within the Sonic fanfic universe.
For Amy, she could become more understanding of the 'evil' or opposing side. Instead of just seeing Metal Sonic as a bad guy, she might learn to look deeper into his existence as a creation. This could make her more empathetic in general.