Who is Eva?

There are many different types of sports. However, this isn't your usual sport. This is a sport dedicated to help exterminate the teens that are unwanted in society. Each person has a strong blood lust. Every year 15 teens are randomly chosen. Now what makes this so different than the hunger games and all those little fairy tales that claim to be tragic, well there are no rules, nothing is in plain sight. Everyone is full of lies. To win this sport all you have to do is kill without getting caught. If you don't kill you will die. If you get caught you will die. There is only one way out and that is to kill secretly. Their are no pacts, their is no truth only duplicity. Well that is what every contestant was told. But what they didn't tell anyone is who was Eva? Why was she able to kill without being detected? What do her letters mean? Can you figure it out? Can someone stop her before it's to late? Can you figure out which contestant is she disguised as? Or is she a contestant at all?