This is a nickname for blood hungry sluts known for snatching up boyfriends. Kallistas usually take them out for coffee or to the movies and convince your boyfriend not to tell you. As the the Kallista gains the mens' trust, she tricks them into thinking that she is better than you and that you're an awful person. Although Kallistas are experts in boyfriend-snatching, they are known to be hideous creatures.
You: My 2 year relationship is over. Out of nowhere, poof!
Friend: Oh my god! Why?! What happened?
You: A Kallista got a hold of him. I wish I could have protected him from it.