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.
When Ruby becomes a demon in this fanfiction, her relationships with her team are bound to change drastically. Her new form and the powers that come with it could make her more unpredictable. Her team members like Pyrrha (if she was still around) might see her as a danger to the team. This transformation could lead to a lot of mistrust and conflict within the group as they struggle to come to terms with Ruby's new state.
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.
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.
Emerald might be a character in this fanfiction. She could start as an enemy, trying to steal the Fall Maiden power from Ruby for Cinder. But as the story progresses, she might see the good in Ruby and switch sides. Neptune could also be there, providing some comic relief in the midst of all the serious Maiden - related business. And Glynda could be involved in training Ruby to control her new powers.
Well, it could be about Ruby facing some kind of extreme situation or transformation that turns her into a more 'red' version of herself. Maybe she discovers a new power within her that has a red hue and she has to learn to control it. This could lead to her having a different fighting style and new relationships with her teammates as they adjust to this new Ruby.
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, one possibility is that Ruby has to face many challenges to become the Fall Maiden. Maybe she has to fight against other powerful foes who also want the power. She might struggle with the new responsibilities and powers that come with being the Fall Maiden.
If Ruby were to become a demon in fanfiction, it would have a huge impact on the story. Firstly, the story would likely take on a darker tone. Ruby, who was once a symbol of hope, would now be a source of fear for some. Her character arc would be completely redirected. She might start to view the Grimm, which are also dark creatures, in a different light. Maybe she would see them as kindred spirits instead of enemies. Her friends would have to decide whether to try and save her or to stop her if she becomes a threat. And the overall theme of the RWBY story about friendship, courage, and fighting for what's right would be challenged as Ruby's new demonic nature tests the limits of those ideals.