Maybe it's a completely new character created for the fanfiction. This new character could be from a far - off land, with a different kind of magic that Voldemort is interested in. Their marriage could be a way for Voldemort to gain access to that new magic or territory.
Another option could be a Muggle - born witch. Voldemort might have been intrigued by her in a different way compared to his hatred for most Muggle - borns. But as his views on pure - blood supremacy become more extreme, she can't bear to be with him any longer, and their relationship ends.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
Harry Potter would likely be one of the main characters. Since Ginny is his love interest in the original series, he would surely be affected by her marriage to Voldemort. He might be the one to try and rescue her or stop the marriage. Ron Weasley, Ginny's brother, would also be important. He would be outraged and would probably be part of any plan to oppose the union.
Snape could also be a character in such a fanfic. His complex relationship with both Harry and Voldemort makes him an interesting addition. He might be in a difficult position, having to keep his cover as a double - agent while also trying to find a way to save Harry without revealing his true intentions to Voldemort.
Ron would also be important. His loyalty to Harry is unwavering. He might be the one who bravely volunteers to go on the most dangerous part of the rescue mission, perhaps facing off against some of Voldemort's most feared Death Eaters.
Hermione is often a very popular character. She can bring her intelligence and magic skills to help Harry in new and interesting ways in the fanfiction world. For instance, she might come up with a unique spell to counter Voldemort's dark magic.
Hermione would be one of the main characters. She might be very skeptical at first about Harry's new friendship with Voldemort. She would constantly be on guard and try to figure out Voldemort's true intentions. Another would be Dumbledore. He would likely be watching the situation closely, trying to guide Harry and also be prepared to deal with Voldemort if he reverts to his evil ways.
In many of these fanfictions, the Marauders (James, Sirius, Remus, and Peter) are quite popular. They were friends of Harry's parents and their presence in a time - travel story can bring a sense of nostalgia and also add a lot of action and adventure. Bellatrix Lestrange can also be a prominent character on Voldemort's side. Her fanatical loyalty to Voldemort and her dark magic skills can make for some intense and exciting scenes, especially when she clashes with the heroes in different time periods.
Hermione could be the main character. She is extremely intelligent and powerful in her own right. She might use her vast knowledge of magic to find a solution that doesn't involve Harry fighting Voldemort. For example, she could discover a way to rewrite the rules of magic that govern Voldemort's power.