Antagonist
Character Role Analysis
Bolter/Growers' Association
Steinbeck made a conscious effort not to demonize either side in this battle, preferring that we draw our own conclusions. But it isn't hard to see who we're meant to like or dislike here. Hey, maybe we're just benefiting from the social legacy of the 1960s, which taught us pretty clearly how to identify "The Man" when we see him.
And Bolter has "oppressor" written all over his nice clean suit. His inability to understand the level of suffering among the workers and his patronizing attitude ensure that he will be getting zero sympathy from Steinbeck's readers.