User Incognito hit the nail on the head. "Street Samurai" was a term used heavily by William Gibson to describe a recurring character of his named "Molly," who also went by "Sally."
I'm not sure if the "Street Samurai" appeared in other writings, but it's a relatively recent term that I found and grabbed onto.
Originally samurai were mercenaries hired by feudal lords to protect assets. Eventually they became wealthy enough to become politically powerful, although it took them a few hundred years to do so. Samurai tribes lived by a strict code and were expected to act accordingly.
A street samurai, then, is a hired mercenary who follows their own code and exists in modern and futuristic storylines.
My character, Katsujirou Kato, is not a street samurai. Another character of mine, "Case," is based on the street samurai model.