Bill Cipher

Bill Cipher is a mind demon, summoned by 'Lil Gideon to steal the combination to Stan's safe, from his mind. He made cameo appearances long before his actual debut and is potentially the hidden and true main antagonist of Gravity Falls.

History
Gideon summoned Bill with an incantation from his journal, upon arriving the demon demonstrated its incredible powers by altering the landscape around it, ripping the teeth out of the mouth of a deer and then undoing the act. He made the deal with Gideon, in exchange for helping him, Gideon would help Bill with some unspecified event that meant a lot to him. He also revaled he had been to Gravity falls before, and implied he had previous dealings with Stan.

Upon checking his Journel Dipper found him described as the "most powerful thing" the writer had encountered, as well as a page covered with crossed out text and blood. Using a spell they followed Bill into Stans mind. There he warned them not to interfer with him, and gave them a demonstration of his power by blasting a hole in Dipper chest and bringing two characters Mable was thinking about to life. Before entering Stan's memories to search for the code Gideon wanted.

Following working out Stan's reasoning, Mable found the memory and Soos offered to destroy it, however it was then revealed to be Bill who had taken Soos form. Now with the code he contacted Gideon and tried to tell him it, however before he could finish, Mable shot the memory into another one of Stans, in which he was standing over a bottomless pit. As such Gideon called off the deal.

Overcome with rage, he trapped them all in a seperate realm and forced them to endure there worse nightmares, before finally planning to kill them. However they were saved by Dipper, who had learned that it was possible to make things come real in the dream universe by thinking them. Using this they battle Bill, and came close to defeating him. However sick of the fight, he simply stopped it and once again rendered them powerless, now in a white void. However because he found them persistant he decided not to kill them, and instead left, but before he did, he warned them of the coming darkness that would destroy life as they knew it. Then he cheerfully left.