Luna Inverse is the first daughter of merchants and elder sister to Lina Inverse.
Luna is the only known Knight of Cepheid, the Flare Dragon Lord of the Red Orb within the Slayers universe. According to Hajime Kanzaka, this makes her exceedingly powerful. So powerful, in fact, she is said to rival Xellos and may even be more powerful than he.
There is much speculation about her looks, power and relationships. One thing is certain: She scares the ever living crap out of Lina.
According to Hajime Kanzaka, Luna's hair color is, in fact, a very dark shade of crimson, possibly due to her affiliation with Cepheid and as a testament to her representation of the Red Orb.
After his resurrection, Dilgear, the half-troll half-wolf beastman, met and was befriended by Luna whom she now calls "Spot". According to the creator, Dilgear detests this name, but after taunting him with fresh meat, he eventually submitted.
Luna does not appear officially in any of the novels and only makes a few appearances in the anime and manga. It is noted that she made an appearance in a radio special in the mid-90's during the time that Lina was traveling with Naga the Serpent. Luna appears in the opening animation sequence of Slayers: Next, Slayers: Try and an eye-catch at the end of Slayers: Next. She makes one known appearance in the special manga runs telling Lina to "Go and see the world. Become a wise adult, Lina."
According to Hajime Kanzaka, Lina went home exactly once after the incident with Luna, saw Dilgear and made fun of him. There is no information on how Lina was received when she went home and no information on whether or not she went back. Assuming this is the case, Lina, at the end of Evo-R, will not have seen her sister since Lina was at least 16 years old. The incident occurring before Lina went to the sorcerer's academy at around age 13.
Recently, Rui Araizumi drew a special Halloween cover of Luna, Lina and their father on a novel that was released in Japan in 2012.