Bill Cipher

"Eat nightmares!"

- 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 true antagonist of Gravity Falls.

History
Gideon summoned Bill in Dreamscapers 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 Journal 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 Mabel was thinking about to life. Before entering Stan's memories to search for the code Gideon wanted.

Following working out Stan's reasoning, Mabel 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, Mabel 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 their 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.

Bill later returned in the Season 2 episode Sock Opera. He attempts to make a deal with Dipper to unlock the password for the computer he found in exchange for a favor. After being rejected the first time, he makes the deal again when Dipper uses all his chances to find the password. After requesting a "puppet" and manipulating Dipper, the two strike a deal and Bill pulls an unsuspecting Dipper from his body. He possesses it and destroys the computer. He then reveals that Dipper had been learning too much and was becoming a danger to him all while revealing his plan to destroy the journal.

After searching for it, Mabel reveals that she is using the journal as a prop in her sock puppet play, much to Bill's delight. He goes there and offers to perform a role in the play in order to get his hands on the book. After Dipper manages to tell Mabel what was going on, she goes to retrieve the journal and almost ruins the play. Bill prevents this from happening and threatens to ruin it anyways if she doesn't give him the journal while reminding her that she constantly ignored her brother to get it to happen anyways.

Mabel refuses his offer and the two fall down onto the stage and get into a fight. She then defeats an unsuspecting Bill by revealing Dipper weaknesses, much to the demon's confusion and dismay. After wearing Dipper's body to the point of unconsciousness, Bill is flung out of it which gives Dipper the chance to take it back. After his defeat, Bill (using the Dipper sock puppet) swears revenge on the twins and states that they cannot stop his plan. Mabel then destroys the puppet by igniting the fireworks it was on and Bill again retreats.

Trivia

 * His name could be a reference to the Beale ciphers, encrypted manuscripts which are said to lead to a great, hidden treasure.