Bill Cipher was a triangular dream demon, formerly existent only in the mindscape before succeeding in gaining access to the real world.
He had been running amok in Gravity Falls, Oregon since being summoned by Stanford Pines over thirty years ago, and was also known for his mysterious demeanor and sadistic humor. Bill is the main antagonist of Gravity Falls , although he doesn't play a central role in the series until the penultimate Season 1 episode " Dreamscaperers.
Trillions of years prior to the events of the series, Bill Cipher originated from the second dimension. Bill despised living there, describing it as a dimension of "flat minds in a flat world with flat dreams. Unfortunately, due to the Nightmare Realm's lawlessness and lack of any consistent physics or rules, it was fated to collapse into itself eventually.
Without a physical form, however, Bill could only access the dreams of the third dimension's beings. In order to make his dealings with mortals easier, he took on the name "Bill Cipher" as his real name would "evaporate one with an expression of horror and ecstasy on their face. When Modoc learned of the prophecy that foretold of an apocalyptic event that would stem from interactions with Bill Cipher, he lit himself on fire in an effort to avert it. The natives eventually discovered a way to defeat Bill by using a zodiac with ten symbols.
They left behind elaborate cave paintings about their encounter with the demon, including how to summon him, and more importantly, a warning never to read the incantation that would summon Bill aloud. In the late twentieth century, a young man named Stanford Pines , who had spent the past six years investigating the town's plethora of unnatural creatures and oddities, hit a roadblock in his research and was left without answers as to how the improbabilities of Gravity Falls had come to be.
During the roadblock, he uncovered the ancient cave containing the ancients' stories of Bill Cipher. Heedless of the warnings, Ford repeated the incantation aloud, summoning Bill into his mindscape. It was Bill who revealed to Ford that Gravity Falls' weirdness was caused by a rift between dimensions, through which the other side's weirdness leaked through.
With Bill's assistance, Stanford drafted blueprints to create an inter-dimensional gateway beneath his home and recruited his college friend Fiddleford McGucket for assistance. As Bill and Ford's partnership seemingly grew to friendship, Ford seemed to develop an obsession with Bill's powers, collecting triangular memorabilia such as rugs and statues, modeling his home's architecture in his image, converting his private study into a place of worship.
Upon his return, an alienated Fiddleford muttered incoherencies before uttering a prediction about "the beast with just one eye. He promptly abandoned the project. The suspicion planted in him by Fiddleford led to a confrontation between Ford and Bill, during which Ford learned that Bill had tricked him, as the portal was meant to act as a gateway to the Nightmare Realm, allowing the demon to bring chaos and destruction into their universe. Horrified and betrayed, Ford shut down the project and attempted to destroy all knowledge of the portal, before accidentally falling through it himself.
Bill either could not see Ford in the thirty years he spent lost in other dimensions or didn't care to, as Ford would remark that they had not seen each other in many years.
With the loss of his human pawn, Bill also lost physical access to the third dimension. Gideon tells him to steal Stan Pines' safe combination directly from his head so that Gideon can steal the Shack's deed. Bill agrees, 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 to whom he is connected , Mabel shoots the memory door into another memory of the bottomless pit with a nyarf dart. Gideon breaks off the 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. 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 representation of the zodiac, almost identical to the one that appears in his entry in Journal 2. Bill Cipher appears again in " Sock Opera ," when Dipper is trying to figure out the password for the laptop from " Into the Bunker. Later, at the moment the laptop is on a countdown for complete data erasure after too many failed password entries, he appears again and Dipper accepts the deal of trading a puppet to obtain the password.
Bill takes over Dipper's body as his "puppet" and destroys the laptop. He then follows Mabel to her puppet show in pursuit of Dipper's journal, which is being used as a prop in the show. After Mabel has been informed of Dipper's body having been stolen by him, he finds her with the journal as she is retrieving it for Dipper. He almost swindles her out of it but ends up in a fight with her. He ends up losing because Mabel uses Dipper's physical weakness and exhaustion to her advantage.
As he collapses, he is thrust out of Dipper's body. Bill possesses the Dipper puppet afterward and says that he will return. However, Mabel activates the pyrotechnics of the play, destroying the puppets and removing any vessel Bill could use to talk to the kids. In " Northwest Mansion Mystery ," he appears on a tapestry hovering over a forest fire with two men either begging for mercy or worshiping him while skeletal corpses are underneath them, foreshadowing bad events.
At the end of " The Stanchurian Candidate ," after a prison-bound Gideon fails to get his father elected mayor, he is shown to have drawn Bill Cipher's zodiac on his cell wall and hidden it behind a cat poster. The symbols on the zodiac have been changed around, and some have been altered. Gideon tears down the poster and completes the drawing by drawing Bill's eye before telling it he's ready to make a deal. In " The Last Mabelcorn ," Bill Cipher appears in one of Ford's dreams, telling him that he's been preparing for "the big day," going on to say that Ford can't keep the inter-dimensional rift safe forever.
Bill throws a copy of the rift down to the ground, opening a red triangular portal in the sky and setting fire to Ford's mindscape. Ford yells at Bill to leave his mind, telling him he has no dominion in the real world.
Bill leaves through the portal, telling Ford that "things change. Far later in the episode, Bill appears in flashbacks as Ford explains his time spent with the demon to Dipper.
In the past, Bill and Ford were partners. Bill could enter Ford's mind at any time, and with his help, Ford built the portal. However, after Fiddleford came out of the portal traumatized by the horrifying things he saw, which were Bill's actual plans, Ford realized Bill had lied to him. He confronted Bill, who revealed his plan to merge the Nightmare Realm he comes from with the real world, causing the apocalypse.
Ford broke off his partnership with Bill then and there, despite Bill's warnings that he couldn't stop the rift from happening. Ford shut down the portal and swore never to trust Bill again. Undeterred, Bill decides to possess someone outside the shack instead. Near the end of " Dipper and Mabel vs. Bill as Blendin convinces Mabel to give him the inter-dimensional rift, which he says can be used to create a "time bubble" that can make time stop.
Mabel gives Bill the rift, which he promptly smashes with his boot. Bill reveals himself and leaves Blendin's body, knocking out Mabel with a snap of his fingers and initiating Weirdmageddon. In " Weirdmageddon Part 1 ," after opening the rift, Bill gains a physical form and imprisons Mabel in a bubble. He introduces himself to the people of Gravity Falls, claiming to be their new ruler as well as introducing his friends to the people. The townsfolk refuse to submit, but Preston Northwest welcomes him instead, offering to be one of his "horsemen of the apocalypse.
Bill recreates the opening theme song in the process. When Bill and his friends prepare to cause havoc, Ford attempts to shoot Bill back into the rift with his quantum destabilizer but misses. When Bill notices Ford and Dipper, he blasts the clock tower. Ford, pinned under rubble, begins to tell Dipper that there's another way to defeat Bill. He is cut short when Bill appears, towering over Ford. Bill captures Ford and presents him to his friends, telling them that Weirdmageddon wouldn't be possible without Ford.
Bill makes an offer with Ford and tells him it's not too late to join his side. Dipper flips through Journal 3, trying to find Bill's weakness. With no answers, Dipper panics, and leaps towards Bill, trying to punch him. In response, Bill knocks Dipper backwards, causing him to fly into a tree. Bill then telekinetically seizes the three journals, burning them in front of Dipper, with two crippled pages surviving.
Bill tells 8 Ball and Teeth that they can eat Dipper for a snack. Bill then converts a random car into a sleek getaway ride for himself and his other friends. The car flies away to the Fearamid leaving Dipper, 8 Ball, and Teeth behind. Bill and his friends are later seen having a rave party inside the Fearamid.
Bill vaporizes the Time Police and Time Baby instantly, and the party resumes. They ask him if he's worried about Dipper freeing Mabel, but Bill says he's not concerned, and that he has "someone on the case. Most of the townsfolk have been petrified and stacked into a " massive throne of frozen human agony " for Bill. He announces his plan to take Weirdmageddon worldwide, and the demons make their way toward the edge of Gravity Falls.
But instead of escaping to the rest of the world, they hit a strange force field-like energy dome that keeps Bill and his chaos stuck in the town. Later in the episode, Bill is angered and unable to understand why he and his friends aren't able to extend their reach and leave Gravity Falls to dominate the world. He realizes that Ford is the only one able to understand the shield and contemplates his next move. He is interrupted by Keyhole , who informs him Gideon, Mabel's jailer, has failed to stop Dipper, allowing him and his group to enter Mabel's bubble.
Bill is not worried by this, calling Mabel's bubble one of his most diabolical traps yet. He asks for Gideon and tells Keyhole to take the day off. Bill tries to persuade Ford by telling offering him unlimited power in a world without restrictions or laws.