Bill Cipher

"I'll be watching you!"

- Bill Cipher

Bill Cipher is a very powerful 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.

He is the final antagonist of Season 1, and is thus far the main antagonist of Season 2.

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 Stan's mind. There he warned them not to interfere 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 Stan's, 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 separate 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 persistent he decided not to kill them, and instead left, but before he did, he warned them of the coming darkness that would change everything they cared about. Then he cheerfully left claiming he would be watching them.

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 but Dipper is too smart to agree. After being rejected the first time, he again offers the deal when Dipper uses all but one of his chances to find the password. Dipper is suspicious of what he wants, but Bill assures him he only wants a "puppet" and manipulates Dipper by exploiting his distain at Mabel ignoring her promises to him and for her weekly obsession. Reluctantly the two strike a deal and Bill then pulls Dipper from his body (revealing him to be the "puppet" he requested). He then possesses it and destroys the computer. He then reveals that Dipper had been learning too much and was becoming more of a threat to his plans, while also revealing his plan to destroy Dipper's 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.

However this backfires, as Mabel comes to her senses 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. In a sense of irony, thanks to all the abuse he put Dipper's body through (and Dipper's lack of sleep) the body weakened quickly. 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 him or his plan. Mabel then destroys the puppet by igniting the fireworks it was on and Bill, again, retreats.

After remembering his past, Old Man McGucket later repairs the laptop Bill destroyed earlier and discovers that the demon plans to bring about the apocalypse by unknown means in order to rule humanity. He attempts to tell Dipper this information at the Northwest party, but is asked by Dipper to not worry about it now. He is shocked to be turned down and panics when he realizes that Bill's plan will be likely set in motion in less than 24 hours.

Trivia

 * His name could be a reference to the Beale ciphers, encrypted manuscripts which are said to lead to a great, hidden treasure.
 * Bill can be summoned if one were to recite an incantation found in Gideon's Journal