Prince of Florin, he becomes betrothed to Buttercup (in the novel on excuse that he will need to continue the royal line because his father is dying). In the novel, it seems initially he might wed the princess of Guilder, but he rejects her because she is ugly and has no hair. He does not care who his bride is, as long as she is very beautiful. Count Rugen then finds Buttercup. The movie never explains how Humperdinck came to choose Buttercup as his bride.
After pursuing Buttercup and her kidnappers using his tracking skills, Buttercup has him swear not to hurt Westley, but instead he lies and has Rugen imprison and torture Westley. It is revealed he hired Vizzini to kidnap and kill her, and now is planning to murder her on their wedding night and framing Guilder like before so that he will have a political excuse for declaring war. Unfortunately his plans are thwarted by Westley, Inigo Montoya, and Fezzik, but they let him live instead of killing him, leaving him to pursue them when they escape with Buttercup.
Although the main antagonist, Prince Humperdinck is incredibly intelligent and has excellent tracking and hunting instincts. He can track the location of animals, and is able to trace events that have happened in a particular place. Buttercup herself says there is no greater hunter than him, proclaiming that he "can track a falcon on a cloudy day."