I found nitara.. I don't know if everyone already did this, which they probably did, but Nitara is in Outworld H-5, inside the Cave where Tanya wishes to trap Liu Kang
I am pretty confident that Nitara is one of the unlockables, because when you speak to her, she simply says "I do not wish to converse with you further, leave me." And when you hit her, she hits you back and says "I will drain your blood for that!!"
It's not. She is there at all times during the day. She actually is part of a quest. You have to talk to a guy around Shao Kahn and he asks you to locate a vampire - you go talk to her - then go back to the guy and mission accomplished.
The Nitara everyone is trying to talk to is the Nitara in Netherrealm who stands side by side Cyrax.