Robert McCall is a former agent with the Company.
The exact identity of this intelligence bureau, also referred to as the Agency, is never specified but there is sufficient evidence pointing towards the CIA, if unspoken. McCall was an agent for many, many years and based on the contacts he has, he has made a big name for himself. Those he made his home in New York City, having a very nice apartment there as well as an ex-wife and a grown musician son, he was a man of the world and saw action in every part of the planet. His knowledge of languages and traditions is impressive.
At the time of the first recorded adventure, McCall is in his late 50's or very early 60's. He is growing extremely tired of the duplicity and the underhanded ways the cloak and dagger game is played and would very much like out. A particularly brutal event in a NY subway in which he promises a desparate man holding a hostage that if the man let the hostage go, all would be fine. The hostage is released and another agent shoots and wounds the man. To keep the wounded and still dangerous man from harming anyone, McCall must kill him. That is the final straw.
McCall turns in his papers and retires. Unfortunately, no one retires from this Agency and his New York boss, known only as Control, lets him know how scared his bosses are. Too much information is in his head, Control warns, to be let free. It was almost certain a hit squad would be dispatched to take him out. "Let them come" was McCall's response. Before too much time has passed, Control lets McCall know he worked out a deal with the upper echelon - let McCall live and maybe he would be there for a favor now and then.
It was not just to get out that McCall retired, though. Not hurting for money, and feeling a need for penance, he began a small one-man crusade to help the underdog get out from under. His reasoning is quite clear to himself - he has done enough bad things on behalf of his "employers" to last several lifetimes and he wants to even things out a bit. This reasoning is not so understandable to some who knew him in the old days. Control, for all his protests against his friend's actions, is one of the few who does comprehend.
"Got a problem? Odds against you? Call the Equalizer" is the advertisement in the newspaper. It gives a phone number in Manhattan where help can be reached, although McCall has obviously learned to never answer the phone directly but let the machine get it and play back messages later. If McCall decides that the caller is legitimate and not, as often is the case, someone looking for a hand out or an acquaintance rubbed out, he will contact the person and schedule a meet in a safe location. Once he takes the case, though, regrets are in store for those he rules deserving of it.
Since McCall is no stranger to the City and many of his former associates, and not a few opponents, live there, he thinks nothing of routinely calling upon them for assistance. Many are less than happy about the calling but some, like his protege and young friend, Mickey Kostmayer, delights in working again with his old mentor and gladly drops whatever he is doing when summoned. Since that is sometimes something Control had him doing, friction can be high between McCall and Control. Nevertheless, the old friendship between McCall and Control goes back a long way and despite his irritations, Control has come to McCall's help himself more than once.