Obviously, Draco Malfoy is a main character. He is central to any story about his collar as it is something directly related to him. Hermione Granger could also be involved. She might be curious about the collar's magic or purpose and try to study it, which could lead to some interesting interactions between her and Draco.
Hermione Granger could also be a main character. She might be the one who discovers Draco in shackles and uses her intelligence to figure out how to free him or to use his situation to gain some knowledge about the Death Eaters. Her presence can bring a different perspective to the story.
Some fanfictions might also involve the villains. Voldemort or his Death Eaters could be interested in the bound collar for their own evil purposes. They might try to steal it or use it against Harry and his friends, which would create a lot of conflict and action in the story.
Draco Malfoy and Argus Filch are the main characters. In these fanfictions, Draco might be exploring the secrets of the collar out of curiosity or for some devious plan. Filch, on the other hand, is protective of his collar as it might hold sentimental value or some power that he doesn't want to be misused. There could also be minor characters like the Weasley twins. They could be causing additional chaos while Draco is dealing with the collar situation, perhaps using it as an opportunity to play more pranks in Hogwarts.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
Draco Malfoy is, of course, the main focus. However, his family members like Lucius and Narcissa Malfoy are often included. They can either contribute to his collapse, for example, through their high - handed family ways, or be part of his support system (although in many cases it's the former). And sometimes, characters like Snape might be involved too, as he has his own complex relationship with Draco and the whole Slytherin house.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.
Obi - Wan Kenobi is frequently in Anakin Snaps fanfiction. As Anakin's master, he has a big role to play. Their relationship is often explored, and Obi - Wan's reactions to Anakin's snapping are crucial. Sometimes he tries to bring Anakin back from the brink, while other times he might be the cause, albeit unintentionally, of Anakin's breakdown.
Well, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
There can be original characters created specifically for the fanfiction. For instance, a detective who is obsessed with bringing down the Joker but slowly starts to lose his own sanity in the process. This character can provide a new perspective on the Joker's reign of terror.
Obviously, the Reaper is a main character. Ruby Rose is also often involved as she is one of the central characters in RWBY and her connection to the concept of death (which the Reaper represents in a way) makes for interesting interactions. Weiss Schnee can also be seen in some fanfictions, as her family's past and her own story can cross paths with the Reaper's storyline in creative ways.