A young and ambitious mercenary who is new to the game but has extraordinary skills. She wants to make a name for herself in the mercenary world and might see the RWBY characters as either rivals or potential allies.
An ex - military mercenary who has his own set of rules and ethics. He might be initially at odds with the RWBY crew due to their different ways of handling things, but eventually finds common ground as they face a common enemy.
A grizzled veteran mercenary who has seen many battles and has a lot of combat experience. He could be cynical at first but slowly warm up to the RWBY team.
It could also be a side character like Penny. If she gets damaged in a battle and is taken to the hospital for repairs. The story could focus on how the RWBY team reacts to her situation, and Penny's own thoughts as she is being fixed. Also, it could explore the relationship between Penny and the people in the hospital who are helping her.
Ruby could be the main character. Since she is the protagonist in RWBY, it would be natural for her to be the one to find the ring. She could be intrigued by its power and try to figure out how to use it for good in the fight against Salem.
A tough and experienced female bounty hunter could be the main character. She could have a no - nonsense attitude and be highly skilled in combat and tracking. Maybe she has a personal vendetta against a particular group in the RWBY universe, like a band of thieves who stole from her family.
Perhaps Professor Ozpin could be turned into a mad scientist in a different take on the story. His vast knowledge and access to resources could be used in a more sinister way. He might start to believe that the only way to save Remnant is through extreme and unethical experiments. Another option could be a young prodigy who is corrupted by the power of knowledge and becomes a mad scientist. This character could be initially a friend of Team RWBY but then turn on them as his experiments spiral out of control.
The main character would most likely be the new thief. They could be an original character created just for this fanfiction.
Most likely Ruby. Since 'rose' is in the title and Ruby is often associated with roses in RWBY. The 'thorn' part could be something or someone that challenges her, like a new enemy or an internal struggle she has to face.
Ruby Rose could be the main character. Her optimistic and fearless nature would make her a great fit for a story involving the Green Lantern. She's always eager to take on new challenges, and if she were to get involved with the Green Lantern's power, she would be quick to try and master it to protect her friends and the world.
Ruby could be a great main character for a Berserker fanfiction. Her innocent and determined nature would be an interesting contrast to the savage and uncontrollable nature of a Berserker. She could be forced to embrace this side of her in order to protect her friends.
It could be an original character. For example, a young girl who discovers she is half - Grimm after an accident. She has to hide her true nature from others while trying to understand her powers.
It could be an original character. For example, a ninja who was born into a minor clan but had great potential. However, due to political intrigue and jealousy from a major clan, he was framed and banished. He then became a mercenary to survive. Another option could be a character who was once a part of a secret ninja organization and was banished when he disagreed with their methods. Now, as a mercenary, he returns to the Naruto world with a vendetta against those who wronged him.