One plot idea could be that Si is a new student at Beacon Academy. They are in awe of Ruby and her skills. So, they start to follow Ruby around to learn from her. Along the way, they get involved in Ruby's adventures against the Grimm, and their own abilities start to develop as they face various challenges side by side with Ruby.
Well, Si might have a unique skill set that Ruby's team lacks. So, when they interact, Si is initially like an outsider trying to fit in. However, as they go on missions together, Si's skills become invaluable. For example, if Si is good at tracking, they can help Blake in finding hidden enemies. With Ruby, they share a mutual admiration for each other's determination. Yang might see Si as a little sibling - like figure and protect them. And Weiss would gradually come to appreciate Si's different perspective on things.
Well, Weiss could be one of the main characters. She might initially be skeptical of this 'new' Ruby but then grows to accept and rely on her. Their relationship could be full of bickering at first but then develop into a strong friendship.
Most likely Weiss, Blake and Yang. They are Ruby's teammates in RWBY and would play important roles in any fanfiction where Si is reborn as Ruby. They would be there to support and fight alongside the reborn Ruby (Si).
I'm not sure specifically as there could be many different versions of such a fanfiction. But generally, it might involve the character Si being reborn as Ruby and then facing all the challenges and adventures within the RWBY universe. Maybe they would have to deal with Grimm, make new friends like Weiss, Blake and Yang, and also attend Beacon Academy.
Well, without having read 'Si Ruby's Twin RWBY Fanfiction', it's hard to say precisely. However, in RWBY, Ruby is a key character. A fanfiction about her having a twin might explore how the twin affects Ruby's relationships with her team, her role at Beacon Academy, and her fight against the Grimm. Maybe it delves into their different personalities and how they complement or clash with each other in the RWBY universe.
If Ruby becomes a Grimm in RWBY, it would be a total chaos. Her friends would be shocked and confused. They might initially try to save her, thinking there's still a chance to reverse the transformation. But she, as a Grimm, would likely have new instincts and powers that are contrary to her former self. Her combat style could change completely, and she might become a threat to those she once protected.
In the fanfiction, Ruby as a demon may have the power of flight. Many demons in fictional works have wings or the ability to levitate. This would give her an edge in battles, allowing her to quickly move around the battlefield. Additionally, she could have the power of regeneration. Demons are sometimes shown as being able to heal from wounds quickly, and this new Ruby might be able to do the same, making her a more formidable opponent.
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.
She might have a more ethereal look. Her hair could start to have a slight greenish tint, like the first buds of spring. There could be a soft glow around her, especially when she uses her Spring Maiden powers.