Bill Cipher

Bill Cipher is a mind demon that can enter a person's subconscious. He can be summoned using an incantation found on his page in Journal 2 under certain conditions: "Triangulum, entangulum. Veneforis dominus ventium. Veneforis venetisarium!", which, translated from its original Latin, comes out to:  "Triangle, I invoke you. I come to the defensive barrier of the mind. I will see the barrier destroyed."

History
At some point before the events of the series, the author of "1," "2" and "3" came across him, recording his powers and appearance in the books.

Bill and the symbol he's based on, the Eye of Providence, appear frequently in Gravity Falls. He was the unseen author of 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, to form the message: "MY NAME IS BILL."

A picture of Bill is shown briefly at the end of the opening theme. A similar picture is seen in the entry about him in book 2. Encircling him are various symbols that 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 "Pine Tree, Shooting Star, and Question Mark" respectively. The entry also makes special note of the glasses symbol, the author of the book seemingly having been alarmed by this. Underneath "Name: ???" is the text: "This odd, triangular being has appeared in my dreams every night for weeks."

His full name appears in his entry in book 3. Behind his name is a message encrypted with a symbol substitution cipher; when decoded it reads: "Liar Monster. Snappy Dresser." There are various notes scattered about the page, such as "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 latter note is crossed out and below another note reading "Can't Be..." can be found written in red ink. On the next page, a diagram of a human head and various aspects of its mind is shown, its top being opened up for a drawing of Bill to enter it. In the lower left corner of the page "DO NOT SUMMON AT ALL COSTS!" is written in large, red letters. The rest of the page is spattered with a dark red substance...

"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, which are not revealed to the viewer. 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 reads the combination out loud to Gideon (who he was connected to), Mabel shoots the memory door into another memory of the bottomless pit with a nyarf dart. Gideon breaks off their deal, to Bill's fury. Bill turns his anger toward 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. And just as though it seems he will be forced out of Stan's head, 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 a wheel almost identical to the one that appears in his entry in book 2.

Personality
Bill is an insane and physically irreverent demon who finds most things amusing (although no one is really sure why). He's outrageous and outlandish, as well as a quick talker and thinker. Though he may come across as simply annoying, he's not to be taken lightly; when angered, he's a force to be reckoned with.

Appearance
Bill is a yellow, triangular creature that bears a strong resemblance to an Egyptian Pyramid and the Eye of Providence. He has a single large eye rimmed with four short lashes on its top and bottom. He moves around largely by floating about, and rarely actually stands on his own two feet. He has thin, black limbs, wears a small, black bow tie, and has a tall, thin, black top hat that he "wears" which floats just above his head. His arms do not seem to be in any fixed position, and can be moved along his perimeter without any difficulty. He has also been seen carrying a small, black cane.

Powers and abilities
Bill's powers are focused around the mind and mental manipulation. He's described as a "dream demon" and displays some of the characteristics of a demon including creating contracts with his summoner. When he is summoned he appears to put everyone and anyone observing into a trance-like dream. The effect is that the world seems to turn colorless, and he can effect the surrounding environment however he likes, as demonstrated by pulling out and returning a deer's teeth. The people observing are unaware they've fallen asleep until Bill leaves and then they realize their eyes have been closed during his whole meeting.

Bill can enter people's minds through their dreams. Once inside a mind he can manipulate anything, including his shape and form and changing the landscape. He can communicate with his summoner like a video call, using his body as a window. In fact he can use his whole body as projector and display images. His body also seems to lead to a pocket space and runs in a slightly different time. (i.e. Mabel jumping into Bill's body and several seconds pass before she reappears, none the wise.)

Once inside of a mind he can also easily pluck information from any others who are also inside and bring those ideas and images to life. The conjured images seem to take a life of their own such as the case with Xyler and Craz.

He also appears to have some form of clairvoyance, describing a darkness coming while projecting an image of the insignia of the insignia of the journals, forshadowing the ending events in Gideon Rises when Stan takes Journal 3 from Dipper and off-screen takes Journal 2 from the pile of things he shakes out of Gideon and is shown to already possess Journal 1 and eventually uses them to complete the machine that the writer of the journals designed but discovered to be too dangerous to use.

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

Quotes
"Deer teeth. For you, kid!"

- Bill Cipher

"It's funny how dumb you are!"

- Bill Cipher

"Remember, reality is an illusion, the universe is a hologram, buy gold, bye!"

- Bill Cipher

"Until then I'll be watching you! I'll be watching you..."

- Bill Cipher

Trivia

 * He is based on the Eye of Providence, which is most commonly seen on the back of the American dollar bill.
 * On the page about Bill in book 2, the -$12 bill says "Semper vigilantum," which in Latin means "always watching."
 * Both journals 2 and 3 contain sections on Bill, but whereas journal 2 contains instructions on how to summon Bill, journal 3 contains instructions on how to stop Bill if he is summoned.
 * Additionally, Journal 2 states that his name is not known, while Journal 3 gives the name Bill Cipher.
 * In the game Rumble's Revenge, one of the translated cryptograms says "i will be returning to gravity falLs …………," foreshadowing his appearance in "Dreamscaperers."
 * When Bill Cipher states that "Reality is an illusion, the universe is a hologram," he is referencing both Einstein and The Matrix.
 * Bill said in Rumble's Revenge that his middle name is Mischief.
 * In Dreamscaperers, when Gideon summons Bill he says some sort of spell or enchantment. In reverse it says backwards message repeatedly.
 * His name is a combination of Bill, possibly referring to his similarities with the eye of providence, which appears on the dollar, sometimes referred to as a bill. The second part of his name is Cipher, which is the algorithm for encryption or decryption.

Bill Cifra Билл Шифр