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.
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.
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.
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.
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.
Obviously, the Reaper is a main character. Ruby Rose is also often involved as she is one of the central characters in RWBY and her connection to the concept of death (which the Reaper represents in a way) makes for interesting interactions. Weiss Schnee can also be seen in some fanfictions, as her family's past and her own story can cross paths with the Reaper's storyline in creative ways.
Ruby is often involved. Since she's the main protagonist in RWBY, it makes sense for her to interact with Nightcrawler in fanfictions. Their different powers and personalities can create interesting dynamics.
Weiss Schnee could also be involved. Her haughty attitude in the beginning could lead to situations where she is humiliated. For instance, if she makes a wrong assumption about someone and is then proven wrong in a very public way.
Often in 'halo scarab rwby fanfiction', Ruby and her friends are main characters. Ruby, as the protagonist of RWBY, has the drive to explore and face new challenges, which the Scarab represents. Halo's Arbiter could also be a main character. His unique backstory and combat skills could add an interesting dynamic. Blake, with her stealth and intelligence, would be important too. She could be the one to gather information about the Scarab or any threats associated with it. And Weiss, with her ice - based powers, could be crucial in combat situations against the Scarab or its related foes.
Weiss Schnee is another character. Given her complex family situation with her overbearing father and the whole Schnee family legacy, fanfictions can delve into how she reconciles her family ties with her own goals and values in the world of RWBY. She might try to break free from her family's negative influence or find a way to redeem her family name.
Ruby Rose is often involved. Her speed and scythe - wielding skills can be really useful on a Dreadnought battleship, whether it's for fighting Grimm or helping with ship - related tasks. Weiss Schnee is also common. Her ice powers can be used for defense or to cool down overheating ship systems.