In some cases, new original characters might be introduced. These could be other hunters or huntresses who have a connection to the source of Ruby's darkness. They could either be helping Ruby or be part of the problem. Maybe they are also victims of the same dark power that is corrupting Ruby and they both have to find a way out together.
Ruby's sister Yang is very likely to be involved. Yang is protective of Ruby, so if the ruby slippers put Ruby in any sort of danger, Yang would be right there. Crescent Rose, Ruby's weapon, could almost be considered a character in a sense in these fanfictions. Since the slippers' power might interact with Ruby's fighting style and thus her weapon. And then there are the villains from the RWBY universe, such as the Grimm, who could be attracted to the power of the slippers and constantly be a threat to Ruby while she has them.
Obviously Ruby herself is a main character. She is the center of the reincarnation concept. Weiss, Blake, and Yang are also important as they are Ruby's team members. Their relationships with Ruby are likely to be explored in different ways in the fanfiction due to Ruby's reincarnation. Ozpin could also be involved as he has a lot of knowledge about the history and secrets of Remnant, and Ruby's reincarnation might be related to some of those deeper mysteries.
Blake is also a common character to be paired with Ruby. There could be fanfictions where they team up for a special mission that is different from the main storyline. For example, they might have to infiltrate a secret organization together, relying on each other's unique skills.
Jaune could be a popular character in such fanfictions. If Ruby is in a relationship with him, he would play a major role in the story. He could be shown as being nervous yet excited about becoming a father. Pyrrha might also be involved in some way, perhaps in flashbacks or as a guiding spirit for Jaune, even though she's no longer alive in the main series.
Well, her team members like Weiss, Blake, and Yang are likely to be important characters. Weiss could be trying to figure out what's wrong with Ruby and how to help her. Blake might use her own experiences with inner turmoil to try and relate to Ruby. And Yang would probably be very protective of her sister and be the first one to jump into action when Ruby is in trouble.
Well, her sister Yang would definitely be one of the main characters. Yang would be very protective of Ruby and would try to help her in any way she could. Weiss would also be important. She might use her intelligence and Dust - related skills to assist Ruby. And Blake, with her stealth and combat skills, could be crucial in protecting Ruby and guiding her through difficult situations.
Definitely her team members Weiss, Blake, and Yang. Since they are always by Ruby's side in the original RWBY series, they would play important roles in this fanfic as well. They might be the ones to first notice Ruby's change and try to help her understand and control it.
Weiss could be one of the characters. Since she's part of Team RWBY and has a complex relationship with Ruby, it would make sense for her to be involved. Another one might be Blake. She has her own story arc and her relationship with Ruby could be explored through these kisses.
Obviously, Ruby herself is the central character. But often, her team members like Weiss, Blake, and Yang are also involved. They play important roles in either causing the situation where Ruby snaps or in helping her get through it.
Weiss could be one. Since she's Ruby's teammate, she might play a role in the fanfiction. Maybe she's initially skeptical about their relationship but eventually comes to support them. Another could be Blake. She could be involved in helping them out in some situations, like when they face difficulties in their married life. And of course, their mentors like Ozpin or Qrow could also make appearances. Ozpin might offer some wise advice about their relationship, while Qrow could be more of a protective figure, especially towards Ruby.