Antagonist
Character Role Analysis
Dan's Fear of Death
The scary Grim Reaper dude who shows up in Dan's recurring nightmares at the novel's start isn't a character per se, but basically, he represents Dan's fear of his own mortality… which is the big obstacle Dan has to surpass to become a peaceful warrior. That's what makes Death the antagonist.
The motorcycle injury Dan suffers is a literal example of him nearly dying at the hands of this foe. After that, the novel reminds us a little more subtly of the Death/death antagonist. For example, when Dan sees the large dark shape of the first man trying to stick him up, Dan briefly remembers the Grim Reaper figure (4.375).
When Dan feels inexplicably endangered in the ocean, he feels Death is swimming beneath him (7.140). It's only when Dan has the final vision of his own demise, his body decomposing across thousands of years, that he realizes “the Grim Reaper, the Death Dan Millman had so feared, [was] his great illusion” (8.67).