Salome is Zetta's former apprentice who gave up her humanity to live in the Netherworld. Despite having immense power she has an unnaturally small amount of Mana power which creatures require to survive in the Netherworld, leaving her body to die slowly. However she intends to keep this a secret from everyone, especially Lord Zetta.
She fell in love with Zetta and vice versa; her wish was to only die in his hands. It turns out the reason why she had a low amount of mana was because little by little she kept transferring it all to Zetta which in turn made him the Overlord he was.
Source: Wikipedia, "Makai Kingdom: Chronicles of the Sacred Tome," available under the CC-BY-SA License.