Bill Cipher

Bill Cipher is a dream demon that can enter a person's mind.

History
Bill, and the Eye of Providence he is based on, appear frequently in Gravity Falls. He was an unseen character who wrote the cryptograms in the online game Rumble's Revenge. His name and existence is deduced by taking all the capital letters in the cryptograms from Rumble's Revenge, forming the sentence: "MY NAME IS BILL."

A picture of Bill shows briefly at the end of the opening theme. This is partially an image from his entry in 2|book 2. Encircling him are various symbols that can link to various characters in the show. Most notably, when he appears physically for the first time, he refers to Dipper, Mabel, and Soos, as "the pine tree, the shooting star, and the question mark" respectively. The entry also makes special note of his association with the glasses symbol. It also says, "This odd, triangular being has appeared in my dreams every night for weeks."

In 3|book 3, the text says "Is he watching me?" and "Bill has proven himself to be one of the friendliest and most trustworthy individuals that I've ever encountered in my life. What a guy! I honestly can't trust him more. Not evil in any way, Bill is a true gentleman." However, the second part is crossed out and below the author wrote "Can't Be..." in red ink . On the next page  "DO NOT SUMMON AT ALL COSTS!" is written in red block letters, the rest of the page splatted with a dark red substance, possibly blood.

"Dreamscaperers" is the first time Bill is directly addressed and introduced in the series. Gideon Gleeful, in an attempt to take over the Mystery Shack once again, summons him. Gideon tells him to steal Stan's safe combination directly from his head, so that Gideon can steal the Shack's deed. Bill agrees once he knows it's Stan, but in return Gideon would have to help him in his own plans. Gideon agrees immediately and the two shake on it. Unbeknownst to Gideon, Mabel, and Soos witnessed the deal in the forest, and get Dipper's help to follow him into Stan's mind.

Once inside Stan's head, Bill confronts the trio and brags that he knows what they're all thinking. To prove his statement, he summons Xyler and Craz from Mabel's imagination. He tricks Mabel and Soos into looking for the memory of Stan's safe combination first, knowing they'd lead him straight to it. At some point Soos is separated from everyone else, and Bill takes his place. Once they find the correct memory he takes it and nearly escapes with it. But while he was reading out the combination to Gideon (who he was connected to), Mabel shoots the memory into another memory of the bottomless pit with a nyarf gun. Gideon breaks off their deal, to Bill's fury. Bill turns his anger towards Mabel and Soos by bringing their nightmares to life, and killing Xyler and Craz.

Dipper returns in a timely manner and tells Mabel and Soos that while they are in Stan's mindscape they can do whatever they want. They battle Bill and he is nearly defeated, but he puts a stop to everything instead. Impressed, he decides to let them go, and after warning them of a mysterious darkness that will change everything they care about, he tells them that he'll be watching them. He leaves in the wheel that encircles him in book 2' s illustration.

Personality
Bill is admittedly insane, offbeat, wisecracking, and generally laid-back. Though he may come across as annoying, his temper is not one to be taken lightly.

Appearance
He's a yellow two-dimensional triangle with a pyramid design. He has a single eye in the middle with eight long lashes. He wears a black top hat that hovers above his head and a bow tie. His limbs are thin and black.

Sightings
The following are sightings of Bill and the Eye of Providence, before his physical appearance in "Dreamscaperers" :

Trivia

 * He is based off of the Eye of Providence, which is most commonly seen on the back of a US$1 bill.
 * On the page about Bill in book 2, the -$12 bill says "Semper vigilantum," which in Latin means "always watching."
 * It is unknown what he wanted in return from Gideon when he got the safe code from Stan's memory.
 * Both Gideon and Dipper's journals contain sections on Bill, but curiously, Gideon's journal contains instructions on how to summon Bill, whereas Dipper's contains instructions on how to stop Bill if he is summoned.
 * The "always watching" phrase, his references to "buy gold" and other common conspiracy theory bylines, and frequent appearences of his likeness reference the Illuminati, a frequent bogeyman of conspiracy theorists that are said to use the Eye of Providence as a symbol of the group.
 * Bill was designed using Phi proportions, a number that approximates to 1.618.