One possible boyfriend for Ruby in fanfiction could be Oscar. In some fanfictions, their relationship develops as they both grow in their abilities and face the challenges in the RWBY world together. They share a bond through their connection to Ozpin, which could potentially lead to a romantic relationship.
Weiss would probably be very protective at first. She has always had a bit of a mother - hen side to her, and seeing Ruby in a more vulnerable, deaged state would bring that out. She would likely try to keep Ruby out of trouble and make sure she was taking care of herself.
A common plot is the 'enemies to lovers' trope. For example, if Ruby's boyfriend was initially from an opposing group or had different beliefs. As they interact more, they start to understand each other and fall in love. This can add a lot of drama and conflict to the story.
One common theme is the discovery of love. Ruby, being a rather innocent and focused on her Huntsman duties character, often has to navigate the new feelings that come with having a boyfriend. For example, in many fanfics, she has to learn how to balance her time between her relationship and her training.
There's a fanfic called 'Ruby's Love Interest'. In this story, Ruby's boyfriend is a transfer student from another academy. It shows how their relationship develops over time, through various adventures and challenges they face together. It also has some really interesting sub - plots involving their friends' reactions to the relationship.
Most RWBY fans don't like fanfics where Ruby is hated. Ruby is a beloved character in the RWBY universe, and fans have grown attached to her positive traits like her leadership, courage, and kindness. So when they come across a fanfic that portrays her in a negative light, they may simply avoid it or feel disappointed that the writer has chosen to go against the general perception of the character.
The other members of Team RWBY would be deeply affected. They would start to question their own abilities and the purpose of their fight. Some might withdraw into themselves, like a broken Weiss who shuts out the world for a while. Yang, on the other hand, could become reckless in her pursuit of vengeance, endangering herself and others. Blake would try to hold the team together, but she would also be mourning privately. The other friends and allies in the RWBY universe would also be in mourning. Qrow might blame himself for not being there to protect Ruby, and Ozpin would see it as a great setback in the fight against evil.
The other characters would be shocked. Weiss would probably be the first to try and figure out what's wrong with Ruby. Blake would be worried about Ruby losing her way, and Yang would be furious. Yang might even go on a solo mission to bring Ruby back, not fully trusting the rest of the team to handle it delicately. The rest of the characters like Jaune and Nora would also be in disbelief and would start planning ways to rescue Ruby.
Yang would be very emotional. She'd be heartbroken that Ruby doesn't remember their shared experiences. She might try to recreate memories, like taking Ruby to their favorite childhood places. Weiss would take on a more practical approach. She'd research any possible cures for amnesia in the RWBY world, whether it's through dust or some other means. Blake would be the quiet support. She'd stay by Ruby's side, listening to her when she's confused and gently guiding her through the things she doesn't understand.
Yang would be completely heartbroken. She'd likely go through a period of intense grief and anger, possibly even blaming herself for not being able to protect Ruby. Weiss would be deeply affected too. She might become more reserved and solemn, vowing to use her skills to uphold Ruby's ideals. Blake would be inspired by Ruby's sacrifice to be more active in fighting against the darkness. She might see it as a call to arms to make Ruby's sacrifice not be in vain.