Ruby could be anyone. It might be a name given to a character in a specific story or just a name of a person in real life who likes to read stories in her own time.
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The Pokémon special, Ruby, was a fictional story created by Japanese manga artist Hiroko Hanami in 1997. The story was about an elf named Ruby who was learning Pokémon knowledge and skills in a mysterious Pokémon training class. Ruby gradually became a powerful Pokémon Master and worked with his Pokémon companions to protect the world from the threat of evil forces.
Well, her sister Yang would definitely be one of the main characters. Yang would be very protective of Ruby and would try to help her in any way she could. Weiss would also be important. She might use her intelligence and Dust - related skills to assist Ruby. And Blake, with her stealth and combat skills, could be crucial in protecting Ruby and guiding her through difficult situations.
I don't know exactly who Ruby is without reading the story. But she is likely the main character, a person who has some kind of story related to reading in the street during her own time.
The relationship between Max and Ruby makes it special. Ruby's caring yet sometimes bossy nature towards Max creates an interesting dynamic. It's a story that many siblings can relate to.