Well, it could be about a character known as the Black Reaper in the RWBY universe. It might involve this character's backstory, like perhaps they were once a normal student at one of the academies in RWBY but something tragic happened that turned them into the Black Reaper. It could also be about their adventures, fighting Grimm or other antagonists in the RWBY world.
Since it's a fanfiction about the Black Reaper in RWBY, the main character is likely the Black Reaper themselves. They would be the central focus of the story, with their actions, decisions, and development driving the plot forward.
You can try websites like Archive of Our Own (AO3). It has a large collection of RWBY fanfictions, and there's a good chance you'll find some great Black Reaper ones there.
One popular RWBY Black Reaper fanfiction might be those that explore the backstory of the Black Reaper. It could delve into how the character came to be and their relationships within the RWBY universe.
Definitely Yang Xiao Long. Her fiery personality and her brawler - like fighting style can create some intense scenes when she crosses paths with the Grimm Reaper. Professor Ozpin is another one. His deep knowledge of the RWBY world and his mysterious past can add layers to a fanfiction involving the Grimm Reaper. Also, Cinder Fall. Her evil plans and her power - hungry nature can make for some great conflict when the Grimm Reaper is in the picture.
One great 'rwby reaper fanfiction' is 'The Reaper's Redemption'. It focuses on the character's journey of self - discovery within the RWBY universe. Another one could be 'Reaper's Shadow', which has an interesting plot that involves the Reaper character in a mystery that ties in with the main RWBY storyline. And there's also 'The Reaper Amongst the Huntresses' that shows how the Reaper interacts with the RWBY girls in unique and unexpected ways.
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.
The exploration of Reaper's backstory in the context of RWBY is also common. Since his backstory in Overwatch is quite tragic and mysterious, fanfiction writers might try to adapt it to the RWBY universe. They could explore how his past experiences in Overwatch might influence his actions and decisions in the RWBY world, such as his views on the concept of heroism and villainy which is very different in the two universes.
Salem would be a great main antagonist in this type of fanfiction. Since she already has a connection to the Grimm, she could use Grim Reaper - like Grimm to further her goals. Maybe she creates a super - powerful Grim Reaper Grimm that can not only kill Huntsmen but also corrupt their souls, making them turn against their own comrades.
One popular theme is the struggle between the Grimm Reaper and the Grimm. It often shows the Reaper's unique abilities in combat against these dark creatures. Another theme could be the backstory of the Grimm Reaper, exploring how they became what they are. Also, relationships within the RWBY universe involving the Grimm Reaper, like friendships or rivalries, are quite common in fanfictions.