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.
Voldemort is often paired with Bellatrix in fanfiction. Their relationship in the original series was one of master and servant, but in fanfiction, it can be explored in more complex and romantic ways. Another character often paired with her is Narcissa. Their sisterly bond can be a central focus, or it can be twisted into something more sinister or complex in fan - created stories. Also, sometimes Sirius Black is paired with her in alternate universe stories, given their family connection and the contrast in their loyalties.
Often, Bellatrix herself is very well - written. Fanfiction authors love to explore her madness and loyalty to Voldemort in great detail. They can really dig into her psyche and show the extremes of her character.
Well, of course, Bellatrix Lestrange herself is a main character in these fanfictions. Harry Potter also often features as he is central to the Harry Potter universe. Sometimes Hermione Granger or Ron Weasley may also be involved, especially if the fanfiction involves the trio interacting with Bellatrix in some way. Dumbledore might also make an appearance as he was a major figure in the original series and could have some influence on the events in the fanfiction involving Bellatrix.
Perhaps a plotline where Harry discovers the truth about his parentage later in life. He has been raised believing Bellatrix is his mother, but then uncovers evidence that it's all a complex web of dark magic and deception. This could lead to a journey of self - discovery as he tries to figure out who he really is and where he belongs in the wizarding world, all while dealing with the stigma of being associated with Bellatrix Lestrange.
Obviously, Harry Potter is one of the main characters. Since it's about his betrothal, he'll be central to the story. The Lestrange he's betrothed to will also be a major character, whether it's an existing character like Bellatrix's younger relative or an original character created for the fanfiction.
Obviously Gibbs himself would be a main character interacting with Harry. He'd be like a father figure.
Snape would have a very different relationship with Harry in this scenario. Instead of being a constant source of disdain, he might see Harry as an opportunity to shape a Slytherin in a new way. Pansy Parkinson could also be involved. She might initially be against Harry being in Slytherin but later come to accept him, or they could be in a constant battle of wits.
Hermione and Betty would be likely to interact. Hermione's intelligence and thirst for knowledge could pair well with Betty's investigative nature. They could work together to solve a mystery that spans both the wizarding world and Riverdale.
Neville could be another character. He might look up to Harry even more in his dragon form, seeing him as an even more powerful protector. Hagrid would be excited to have a dragon - Harry around. He could help Harry understand his new dragon nature. And some of the Slytherins, like Draco, might be jealous or scared of Harry's new form.
Obviously, Harry Potter himself. And there could be some characters from the original series who play important roles in helping Harry understand his new identity, like Hermione and Ron.