Ruby has bright scarlet skin, burgundy irises and dark burgundy hair. She is slightly taller than Steven and has a thick build. She wears a maroon tank top and a matching colored headband around her thick and fluffy, outgrown hair cropped at the neck. Her ensemble includes mid-thigh burgundy shorts, and short, pointed maroon boots similar to Amethyst's. Her gemstone is located in the palm of her left hand and has a square facet.
Approximately 5,750 years ago, Ruby lived on Homeworld as a common foot soldier.
Homeworld was attempting to create a new colony on Earth, which was overseen by Blue Diamond. She was part of the diplomatic team sent to stop Rose's Rebellion on Earth, as one of the three Rubies assigned to guard Sapphire, an aristocratic Gem with prophetic vision, adamant for destroying the uprising in its crib. When the rebels attacked, the three Rubies fought back, attempting to fulfill their jobs, only for two of them to be poofed by Rose Quartz and Pearl before turning to Sapphire.
Before her imminent poofing, Sapphire thanked Ruby for her service, at which point Ruby realized Sapphire had already predicted this and that she knew her physical form would be destroyed. Unable to accept that she had failed her mission of protecting her, Ruby tackled Sapphire to defend her from Pearl and caused them to accidentally form Garnet, changing the prophecy foretold by Sapphire. They immediately defused and were surrounded by an angry crowd. Furious that Ruby interfered with the outcome of the battle (and participating in an unsanctioned fusion with a member of her court), Blue Diamond ordered that she be shattered. However, Sapphire grabbed Ruby and jumped out of the Cloud Arena towards the Earth below, landing them softly with her levitation.
Ruby was distraught that she was unable to protect Sapphire. When it began to rain, she carried Sapphire to a cave for shelter, where she saw Sapphire's single eye for the first time. The two talked about their experience and feelings being fused as Garnet.
Ruby and Sapphire grew closer and eventually decided to fuse again. Garnet was found by Pearl and Rose shortly after forming. Upon being met with acceptance and not revulsion, she began to bombard Rose with questions about the events and circumstances that allowed her to become Garnet, as well as her own feelings about existing as a fusion. Rose told her to "never question this", and said that she was already the answer to all her questions—love. This would be the beginning of Ruby's existence as part of Garnet for the next few millennia, fighting the Gem Homeworld with the Crystal Gems to protect Earth.