Karen Parker is Alex Mercer's ex-girlfriend in video game, Prototype.
Alex's sister, Dana, directs him to Karen as she needs rescuing from the military.
When Alex arrives in her apartment, Karen is surprised to see him as alive as she thought he's dead. He tells her they need to leave the place, and she needs a military vehicle for her rescue.
Since then, Karen aids him in stopping the virus by getting him into collecting genetic material. However, while collecting it in an infected building, Specialist Cross shows up and tells Alex that he should choose his friends carefully. This led him to realize that Karen has betrayed him, and is working for Blackwatch. When he tries to confront her, she left.
After absorbing the scientists, Alex locates Karen and her fate is unknown.