The strange anachronism of the new Middle Ages in America means cowboys, pirates, and knights all coexist, sometimes interacting. Often mentioned in these schoolyard debates, but somewhat way off in the distance, is the ninja. In feudal Japan, they were assassins--not the mystical, black shroud beings of video games, but spies, who's fabled "invisibility" comes from their tendency to blend in with the lower classes in hierarchical societies.
In America, "Ninjas" is largely a catch-all term for fighters trained in Eastern-style martial arts, with modicum of armor or weapons. Ninja is used to refer to anyone who is expertly trained in hand-to-hand combat, which is usually a mishmash of kung-fu, karate, jujistsu, even non-Asian arts like Capoeira. Anyone who trained at a dojo is called a ninja, and anywhere a ninja trains is called a dojo. Dojos aren't particularly common in the Heartland--they're mostly found in the coasts, particularly the Pacific Coast, where they have much of of a connection to East Asia, and where the upper classes who don't engage in the feudal order might want to prove their fighting bona fides.
For the most part, "Ninja" is an honorary, and in America, much more style than substance. However, they are orders of undetectable assassins who sometimes pretend to be the more conspicuous ninjas. it's much easier to surprise people who think you're actually playing by the rules.