Antagonist
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Character Role Analysis
Kasper Gutman
The actual antagonist of this picture changes constantly. Initially, Brigid (as Ms. Wonderly) sets up Floyd Thursby as the nemesis, but Thursby never appears on camera. Then, Brigid sets up Gutman as the antagonist, the "fat man" pursuing her. Gutman does drug Spade and allow Wilmer to kick him while he's down, so he might be the least honorable of the bunch.
But the antagonist could also be the greed that drives all the characters toward the Falcon. When the Falcon turns out to be fake, you can practically hear greed gloating, and Gutman, consumed by greed, rushes off to continue his journey for the Falcon.