Antagonist
Character Role Analysis
Tristero
Okay, Tristero isn't actually a character, but it/he/she/them is still the antagonist in the book. After all, it makes our gal Oedipa very unhappy.
It is not impossible that Oedipa imagines the Tristero conspiracy, or that (even if it exists) it's not half as bad as she thinks it is. But Tristero is her absolute obsession. When she first hears the word in Wharfinger's play, "the word hung in the air as the act ended and all lights were for a moment cut; hung in the dark to puzzle to Oedipa Maas, but not yet to exert the power over her it was to" (3.130). In short, Oedipa is tormented by the idea of the Tristero, and her desire to understand it sends her life spinning into confusing madness.