Cassandra Crossing is an agent with American Intelligence.
That is about as close as I can come to who she works for. Or at least used to work for. When we meet her, her covert operating days have come to a loud and bloody end thanks to a shootout in South America. That and the fact that her father chose to run for President and quickly became the front-runner. With her picture and that of her twin brother plastered everywhere on newspapers and television and the Internet, anonymity is not an option.
Crossing is in her [likely] early 30's when the first recorded adventure happens. Her younger twin, Cal, had chosen to go into politics like his father and was already a Congressman. Crossing had no such interests and far preferred the excitement and the danger of secret intelligence work. She certainly did not need the employment to pay the bills, being quite wealthy through inheritance, but chose it because it not only excited her, it gave her purpose.
She is described as having a "five foot ten athletic frame [which] moved with the grace of a jungle cat. She gave off an air of raw physical power and in her case it was not false". We learn of her exceptional fighting ability early on in the series when she recognizes she is being set up for a mugging but a man and woman team and almost gleefully walks into it, taking them both down in seconds leaving one with a broken jaw and the other a smashed testicle, among other discomforts.
Crossing has a sharp mind and a strong opinion and no qualms about backing up her beliefs with actions.