Karen is the ADA "attached" to the NYPD SCU, and into the job with every fiber of her being. With her fierce dedication to justice, she is the ideal person for a job that requires her to deal with the worst of humanity, but for most people, staying on her good side means staying away from her, period. With what little spare time she has, she actively promotes the cause of Metahumans - Karen herself is one, with the power of precognition. Her more secret obsession is tracking down the cases that "got away", most significantly Mark Simmons.
Her presence as both friend and unknowing antagonist of the team is a key part of the interpersonal dynamic of Professionals.