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.
In addition to Amy Rose, there could be a group of antagonists. They might be servants or allies of the 'angel of darkness'. On the other hand, there might be some side characters from the Sonic universe like Knuckles. He could be involved in the story either as an ally of Amy Rose or in some sort of conflict related to the 'angel of darkness' situation.
Well, a frequent theme is transformation. In this fanfiction, Amy Rose could be in the process of changing due to the 'angel of darkness' element. It might be a physical transformation or a change in her personality. Also, the theme of mystery is likely present. The nature of the 'angel of darkness' and its connection to Amy Rose would be full of mystery, making the readers keep on reading to find out more.
Another possible main character could be her rival. This rival could be from a different boxing school or have a completely different boxing style. Their interactions both in and out of the ring would be a major part of the story.
Even one of her friends like Tails could be the betrayer in a really unexpected plot twist. Maybe Tails is forced to betray her by some external threat. For example, if an enemy captures Tails and threatens to harm him or Sonic if he doesn't give away some important information about Amy Rose or lead her into a trap.
Amy Rose is one of the main characters. She brings her own personality traits like her spunkiness and loyalty. The human, who is not clearly defined yet as it's a fanfiction concept, is the other main character. They are probably going to be the character through which we see the interaction between the Sonic world (represented by Amy) and the human world. Their relationship will be at the core of the story, with their different backgrounds and natures creating interesting dynamics.
Well, Amy Rose and Sonic are the central figures. Amy Rose might be the driving force in some fanfictions, as her feelings for Sonic can lead to different story arcs. Sonic is the one who usually jumps into action. Additionally, in fanfictions related to Sonic and the Black Knight, characters such as Merlina can also be significant. She has her own motives and powers, and her interactions with Amy and Sonic can add depth to the story.
She could meet an alternate version of herself. This other Amy Rose might have different values or goals.
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.
In certain fanfictions, a jealous rival of Amy Rose could be the kidnapper. This could be another female character who has feelings for Sonic and sees Amy Rose as a threat. She might think that by getting rid of Amy Rose, she can have Sonic all to herself. Or it could be a character from Amy Rose's past who has some unresolved issues with her and decides to kidnap her to get revenge or make her pay for something that happened a long time ago.
Another option is that Metal Sonic could be the main antagonist. He might see Amy Rose's new bad side as an opportunity to create chaos and undermine Sonic. Metal Sonic could manipulate Amy Rose further into doing more evil deeds, all while trying to frame Sonic for her actions.