Maybe a new character, a dark wizard who was once a student at Hogwarts but turned to the dark side due to some past traumas. This wizard could be very charming and use his charm to make Harry fall for him/her, leading Harry to consider the dark side as a place where he could be with this person.
It could be Bellatrix's younger sister who has a different view on the dark side compared to Bellatrix. She might be more conflicted and her relationship with Harry could start as an enigma that draws Harry closer to the dark side.
There could be a half - blood witch on the dark side who is actually a descendant of one of the founders of Hogwarts but was shunned by the good side. Harry, out of his sense of justice and growing attraction to her, decides to join the dark side to be with her and fight for her rights.
Well, one possibility could be that Harry is drawn to the allure of the dark side because of a love interest there. Maybe a mysterious dark witch or wizard catches his eye and through that relationship, he starts to see the dark side in a different light. For example, they might introduce him to a more 'liberating' way of using magic according to the dark side's view, which slowly makes him question his previous beliefs.
It could be Draco Malfoy. In this fan fiction, Draco might have a change of heart towards Harry when he becomes a Slytherin. Their initial rivalry could turn into a deeper connection, with Draco seeing Harry in a new light and falling in love.
It could be Dumbledore. In a twisted take on the story, if Harry joins the dark side, Dumbledore would be the one trying to stop him. Dumbledore has always been the guiding force for the good side, and Harry's defection would be a huge blow. Dumbledore would then have to face the difficult decision of either trying to save Harry or stopping him by any means necessary, making him the antagonist from Harry's new 'dark' perspective.
Well, perhaps Harry gets tired of being constantly in danger and feeling unappreciated in the wizarding world. He might be lured by the Dark Side with promises of power and safety. Voldemort could offer him a place by his side, showing him a different view of magic that seems more appealing in a warped way.
In 'Harry Joins the Dark Fan Fiction', perhaps Harry is deceived by a powerful dark wizard who convinces him that the dark path is the only way to truly protect the wizarding world. This could lead to him abandoning his previous values. He could be trained in dark arts, and as a result, he might become an outcast among those he once called friends. However, there could also be a plot twist where he realizes his mistake and tries to redeem himself.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.
Obviously, Harry Potter himself would be the central character. Then, Voldemort would be a major figure as he is the main representative of the Dark Side. Bellatrix Lestrange could also play an important role as she is one of Voldemort's most loyal followers. She might be the one to initially tempt Harry or guide him into the ways of the Dark Side.
It could be the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
Perhaps Harry discovers some dark secrets about the wizarding world that make him lose faith in the good side. For example, he might find out that some of the so - called good wizards have been involved in immoral acts. This disillusionment leads him to think that the dark side might be a better option. As he joins them, he starts to abuse his power, using dark magic to get revenge on those who he believes have wronged him.
Hermione could be a likely romantic interest. She is intelligent, kind - hearted, and has always been there for Harry. In the context of Harry being abused, Hermione's understanding nature would make her a great support. She could help him heal from his past traumas. Another possibility is Luna. Her unique perspective on life and her gentle soul might attract Harry. She could offer him a different kind of comfort, one that is more ethereal and accepting, which would be very beneficial considering his abused past.