Voldemort himself, of course, is the most prominent in a sense. But in the fanfiction, his relationship with the other Death Eaters could be different. He might be more distrustful or have different expectations of them. And then there's Barty Crouch Jr. He could be a major player in the fanfiction, with his skills in disguise and dark magic. His actions could drive some of the major plotlines in the fanfiction.
The Hogwarts students would be divided. Some would be in disbelief and would still hope that Harry has a good reason for joining. They might secretly support him. Others, especially those who have lost family members to the Death Eaters, would be furious and see Harry as a traitor. Harry's old enemies at Hogwarts, like Draco Malfoy, would also be confused. Draco might initially think that Harry has finally seen the 'right' side, but as he realizes Harry's true intentions (if they are good), he would be conflicted as well since he's been brought up in a Death Eater - influenced family.
The wizarding world would be in shock. Harry was seen as the 'Chosen One' to defeat Voldemort. If he joined the Death Eaters, the Order of the Phoenix would feel betrayed. They might try to rescue him or think he's been an undercover agent all along. Hogwarts would be in chaos, and students would be confused and scared.
A possible motive could be self - preservation. If they believe that Voldemort's downfall is inevitable and that by saving Harry, they can gain some form of leniency from the side that opposes Voldemort. They might think that Harry has enough influence to spare their lives if they help him.
The Order of the Phoenix would see Harry's joining of the Death Eaters as a major setback. They would likely view it as a sign that Voldemort has become even more powerful. Dumbledore, if he were still alive, would be very concerned and would try to understand what led Harry to this decision. He would also be devising a plan to get Harry out of the clutches of the Death Eaters.
Some Death Eaters might be angry. They have a certain self - image and ideology in the original story, and if fanfic distorts that in a way they don't like, they would not be happy. For instance, if a fanfic makes them out to be weak or cowardly, they would likely react negatively. However, others might be more open - minded and see it as an interesting alternative view of their world, but this would be a minority among them, as they are generally quite set in their ways.
There's no way to know for sure without reading the fanfic. It could be some well - known Death Eaters like Bellatrix Lestrange, Lucius Malfoy, and Severus Snape (although Snape has a more complex relationship with Harry in the original story).
Perhaps there is a prophecy that the Death Eaters misinterpret at first. They think killing Harry is the right thing, but later they discover that saving him is actually crucial for their own survival or for some greater good in the wizarding world. So they decide to go against Voldemort's orders and save Harry. For instance, they find out that Harry has some power that can prevent a much greater evil from emerging, which would also threaten the Death Eaters.
She usually resists. Hermione is known for her strong will. So in most of these fanfictions, she'll use her intelligence and magic skills to fight back. For example, she might use clever spells to escape or mislead the Death Eaters.
It's also possible that some of the Death Eaters themselves could be main characters in a sense. For example, Bellatrix Lestrange could be a central figure if the fanfiction focuses on the power struggle within the Death Eater ranks during Harry's hunt, or if there is some backstory or different take on her relationship with Voldemort and the other Death Eaters.