Disclaimer: I don't own Sailor Moon, or Setsuna
Her true name was Tokiko. Her name fit her, as she was Time's child. A child born of necessity. The need to guard life, and make sure it progressed appropriately. Though she looked of 21 years, she was 15. 15 billion years ago she was created. Created, not born. Her only parents were Time, Space, and the Universe. The Big Bang caused her creation; her very existence was just a fact of fate.
For three billion years, she walked the edges of the universe. It was not dark in the universe, for how could it be dark, when it had never had light to compare to? It was not describable. You can't describe the feeling of knowing that you have eyes, a nose, a mouth, and ears but you don't know what they are for. There was nothing to see, nothing to smell, nothing to taste, nothing to hear.
One day, this all changed. Slowly, she noticed a large shape. A shape that was different, for she could see it. Light had entered the universe. The sound of crying reached her ears, and she heard the noise of her future companions. Floating by the light source, were two tiny girls. One was white; her pale hair shimmered in all the colors that white light could be split into. She was named Prism. Prism's twin was her opposite. Her skin was dark, her hair almost black. She was called Absence. For Tokiko had learned of the absence of light.
Over the next 12 billion years, she raised the two children, her sisters. Together, they watched the universe expand, and fill up with stars, galaxies, planets. On these planets, the three inhabitants of the universe found life. While Prism and Absence wandered the galaxies, making sure that light never faded from the universe, Tokiko watched over the evolution of cells to the humans that formed the Silver Millennium. Knowing that her duty lay there, she took the name of the warriors, the Sailor Senshi.
Tokiko took two new names during that time, one as Sailor Pluto, the warrior who came from the planet that everyone thought was empty. In truth, No one lived on Pluto, but a carefully made story of how the plutonians never wanted to be seen gave Tokiko her first new name. Her other name was Sakisaki. She chose that name, for she could see into the distant future. This name was so that she could travel to the other planets, doing her duty of making sure Time stayed correct, without being noticed.
During this peaceful time, Prism and Absence traveled to Alpha Centauri to check on its slightly fading glow. Soon after they left, the Silver Millennium fell, and when they returned 10,000 years later, their older sister, Tokiko, was living on the planet Earth, under a new name. Her new name was Setsuna, for that was what this time period felt to her. In her 15 billion year life span, these 10,000 years were nothing but a small moment to her.
No one knew her true name, her true origin, or her true family. But she had never told them. She felt no need, for they were only a moment in time, and would fade away in a 1,000 years.