Khara worked for Aria, which she forgot about.
Chapter 12 reveals that Aria was the "queen of the lulus of East Asia." Given that she and Aria are both presumably over the age of 100, then that could mean that Aria had Khara as a servant long ago whom she had a one-time relationship with. She then made her into a vampire most likely as a promise that they'd always be together...but through whatever means, she forgot about it. She could've also used the spear as part of the promise in some way.