Luna, looks 20 years old, but she is actually a very young girl, 8 years of age !
When she was 3, unknown people from a secret Laboratory murdered her parents and abducted her to use in their experiments. They linked her soul with a old witch's soul and locked on a weapon (Zampakutho, this is in the RP story), they also did other experiments which accelerated her growth (that's why she seems older).
But, the spirit of the mysterious witch was too strong for the little girl, and the witch can control her body at anytime.
A long time ago, this "witch" was a good woman, and she loved a man who was an enemy soldier. One day, they are found and and killed by their enemies. She fused her soul into a weapon to bind her to her lover for eternity ...
She hates humans, but she hides her pain and saddness with a derisive and sadist mask.
Many people think she is bad, a "bitch", but nobody knows her true story ... nobody understands her.
In the RP, a man who was also a test subject from the same laboratory (Raiden from Metal Gear infact), tried to help the little girl Luna, but even he doesn't really know it is a spirit who actually control the body of the girl ...
Other details : She control the power of Ice