Bill Cipher is a two-dimensional interdimensional dream demon from the now destroyed dimension, Euclydia. Formerly existent only in the Mindscape, Bill briefly succeeded in gaining access to the real world and a physical form. He had been running amok in Gravity Falls, Oregon since being summoned by Ford Pines over thirty years ago. Known for his mysterious demeanor and sadistic humor, Bill is the main antagonist of the overall series.

He does not play a central role in the series until his first physical appearance in "Dreamscaperers", though he is pictured throughout the entire series, such as the rug in the Mystery Shack, the window in the building's attic and is pictured in the title sequence. His impact on those he's encountered has changed them for the better or worse.

He was voiced by series creator Alex Hirsch, who also voiced Stanley Pines, Old Man McGucket and Jeff in the same show, as well as the Curator in Amphibia.