was he a realist or naturalist
Stephen Crane was a naturalist a lot like Jack London. His main themes were man vs. nature and man vs. himself in which nature is an abstract, omnipotent force that does not (and really cannot) care what becomes of the individual. In Crane's writings, man is often faced with strengths and weaknesses within himself when confronted by extremes of uncaring Nature (see the short story The Open Boat).