It's possible that there are some new characters created for this fanfic who are the abusers. They could be some rogue wizards who want to use Harry's fame for their own gain. Bellatrix, for some reason, decides to step in and save Harry from these unknown and dangerous wizards.
Maybe in this fanfic, some of the Death Eaters who don't like Harry's potential power could be the abusers. They might try to harm Harry to prevent him from growing into a powerful wizard. And Bellatrix, having a change of heart, decides to oppose her fellow Death Eaters to save Harry.
The Dursleys are the most likely abusers as in the original Harry Potter story, they were the ones who mistreated Harry. They could be the ones from whom Bellatrix saves Harry in the fanfic.
In the fanfic, Bellatrix might use her magic to protect Harry from his abusers. Maybe she realizes the wrongness of the abuse and decides to be his unexpected savior. She could use her dark magic skills to intimidate those who harm Harry, creating a sort of dark - but effective - protection around him.
It could be an ancient dark magic artifact. The artifact has a hold on Bellatrix, and in a way, it's the 'enemy' that Harry has to defeat to save her. The artifact might be trying to use Bellatrix to carry out some dark plan, like opening a portal to a world full of dark creatures. Harry has to figure out how to break the connection between Bellatrix and the artifact while also fending off the artifact's attacks.
The obvious antagonists would be the Death Eaters. They are Voldemort's followers and are often causing trouble in the wizarding world. In this fanfiction, they might be the ones who have put Cedric in danger, either to lure Harry or for some other dark purpose.
Voldemort would be a likely character to interact with Harry. Since Bellatrix was one of Voldemort's most loyal followers, he might take an interest in Harry as her son. He could either see Harry as a potential powerful ally or a threat depending on how his powers develop. Another character could be Draco Malfoy. Given the connection between the Malfoy family and the Death Eaters, Draco might have a complex relationship with Harry, perhaps initially seeing him as an equal in the dark magic world but later having doubts as Harry shows signs of not fully embracing the dark side.
The Death Eaters are a likely choice. They might target Susan Bones because of her family's connection to the wizarding world. For example, her aunt Amelia Bones was a well - known figure, and the Death Eaters could be after something related to her or the Bones family in general.
Dark wizards or witches who are on a power - hungry spree could be the antagonists. They might see Daphne as an easy target or a way to get to Harry Potter himself. So they kidnap her to lure Harry into a trap.
In a 'Harry Potter Saves Bellatrix' fanfiction, perhaps Bellatrix is in a situation where she's about to be killed by other Death Eaters who think she's a liability. Harry, having a sense of justice and seeing the potential for redemption in her, steps in to save her. He could then try to show her the error of her ways, using his own experiences of losing loved ones to Voldemort's ideology. Bellatrix might initially resist, but gradually come to understand Harry's point of view.
Obviously, Harry Potter himself is the central character as the one being abused and neglected. The Dursleys are also key characters as they are the ones who abuse and neglect him in the original story. Hermione and Ron might also be involved as they could be the ones to discover Harry's situation and try to help him. Dumbledore could also be a part of it, perhaps as someone who failed to notice or prevent Harry's abuse initially.
For Bellatrix, she could start to question her loyalty to Voldemort. Her interactions with Harry might make her realize that there's more to the world than just blindly following the Dark Lord. As for Harry, he could become more aware of the true nature of the Death Eaters at a very young age, which would shape his future battles. He might also develop a sense of pity for Bellatrix in a way, understanding that she's a victim of Voldemort's ideology too.
They might face magical justice. In some fanfictions, the Ministry of Magic could get involved and strip them of their magic, or at least put them under strict surveillance.