Without reading the fanfic, it's hard to say for sure. But it could be the main characters from RWBY like Ruby, Weiss, Blake or Yang. Maybe it focuses on one of them in particular as they face a difficult situation in the 'darkeness'.
Since it's a RWBY fanfic, it's likely that at least some of the main RWBY characters are involved. However, it could also introduce new characters that are relevant to the story of 'writhing in the darkeness'. It might center around a character who is not typically in the spotlight in the official RWBY series and show their struggle in this dark scenario. For example, it could be about a minor character from Beacon Academy who gets caught up in a dark mystery and is 'writhing in the darkeness' as they try to figure things out.
I'm not sure specifically as there could be many different interpretations. It might be about a character in the RWBY universe writhing in some sort of darkness, perhaps a physical or metaphorical one. It could involve a struggle, an emotional turmoil or a dangerous situation set within the RWBY world.
Obviously, Artyom would be one of the main characters. And then there would likely be the main RWBY characters like Ruby, Weiss, Blake, and Yang. They might interact with Artyom in different ways. For instance, Ruby could be curious about Artyom's weapons or his background.
It's also possible that the fanfic could focus on side characters from RWBY but in a new light related to the'shine' concept. For example, Penny could be a central character in the 'rwby shine fanfic' if the story is about how her robotic nature can shine in a different way, like her circuits starting to glow with a new kind of energy that changes her abilities and her role in the RWBY universe.
Well, of course, Emiya is one of the main characters. He brings his own set of skills, like his swordsmanship and his ability to project weapons. Then there are the RWBY characters such as Ruby Rose. Ruby is a key figure as she is the leader of Team RWBY. Her innocent yet determined nature often clashes or combines interestingly with Emiya's more stoic and experienced personality.
Well, it likely still features the main characters from RWBY like Ruby, Weiss, Blake, and Yang. But in the context of redemption, perhaps there are new characters or side characters who play a crucial role in their journey towards redemption. Maybe an old enemy who now has a change of heart and becomes an ally.
I assume Rose and Bird are the main focus. However, in RWBY - related fanfics, there are often other characters from the RWBY universe that interact with the main ones. So perhaps characters like Ruby, Weiss, Blake, and Yang could also make appearances in 'rosebird rwby fanfic' and play supporting roles or be part of the overall plot in some way. Maybe there are also antagonists in the fanfic that are either new or based on existing RWBY villains.
There could also be new characters introduced in the RWBY Gods fanfic. Maybe a character who has a special connection to the Gods, like a prophet or a chosen one from a long - forgotten tribe. This character could play a crucial role in bridging the gap between the RWBY characters and the Gods.
Most likely, the main characters would be Ruby, Weiss, Blake, and Yang, as they are the central characters in RWBY. In the 'gamer fanfic' version, they would be the ones experiencing the game - like elements.
Typically, Ruby, Weiss, Blake, and Yang are still main characters in 'immortal rwby fanfic'. Their characteristics might be enhanced or changed due to the immortality factor. For instance, Ruby's leadership skills could be further developed over time. Weiss may use her long life to master more complex magic. Blake could have more time to fight for equality in different historical periods, and Yang's strength and determination might reach new levels as she has all the time in the world to train.
It's hard to say without actually reading the fanfic, but most likely the main characters from RWBY, like Ruby, Weiss, Blake, and Yang, would be involved. They might be the ones dealing with the new nuclear elements introduced in the fanfic.