Bill Cipher/Synopsis

Before Gravity Falls
Little was known about Bill before his time in Gravity Falls, although there are hints about his past, including the statements made by the show's creator himself. Trillions of years prior to the events of the series, Bill Cipher lived in a second dimension, which he however despised living there, describing it as "flat minds in a flat world with flat dreams." Bill eventually burned it along with everyone he's ever known, including his own parents. He later took over a boiling, shifting intergalactic foam between dimensions—a lawless, unstable crawl space known as the Nightmare Realm, and became a leader of a gang with many monsters who obeyed his will. Unfortunately, the Nightmare Realm was fated to eventually self-destruct due to it's lawlessness and lack of any consistent physics or rules. After learned about a prophecy that stated he could merge the Nightmare Realm with the third dimension, Bill manipulated countless humans over the time in order to become a physical being and liberate that dimension as well.

In the episode Weirdmageddon: Part I, Bill reveals he has been trapped in a decaying dimension, "waiting for a new universe" to call his own, for one-trillion years (this dimension was later stated by Bill to be the 2nd Dimension). Ford Pines, Bill's main enemy, stated in an earlier episode that he's older than the Milky Way (which is 13.2 billion years old). Assuming that he's old enough to have recalled one-trillion years of mental torment, this would mean he's older than our dimension by at least 987 billion years. In that same episode, he also referred to the populace of Gravity Falls as "single lifespanned", meaning he may have had numerous lifespans, and he may have either lived through those 1 trillion years in different lifespans, or he may have had lifespans before his time in the dimension. Other than his possible age, his "early life" (if whatever he is even has a definitive "life"), is completely unknown.

Before his debut in the episode Dreamscaperers, Bill Cipher made a cameo in the background of numerous episodes as drawings on the walls, card backings, and even in a single frame of the opening sequence, and being alluded to by series creator Alex Hirsch for the length of the season, before finally debuting in the penultimate episode.

Season 1
In that episode, the first season's main antagonist, Li'l Gideon, summoned him using a ritual found in Journal #2 to have him steal the combination to Stan Pines' safe via invading his mind. Bill made a deal with Gideon, saying that if he found the memory containing the combination, Gideon would help him with his plans, probably by Bill possessing his body. Later on, after the protagonists found their own way into Stan’s mind with Journal #3, they met Bill where he warned them to keep out of it, and showed his strength by blasting a hole in Dipper’s chest, sending Mabel a few seconds into the future, and blinking two characters she was thinking about into existence before flying into the shack in Stan’s mind. The trio followed him to try and find the memory before he could, and when they did Soos offered to destroy it with his strength, but revealed he was actually Bill in disguise, who took the memory to read it to Gideon.

However, Mabel and Soos caught up with him while Dipper looked for a personal memory, being able to knock the memory into the Bottomless Pit from the episode of the same name through one of Stan’s other memories. Seeing this, Gideon called off the deal and switched to “Plan B”, which enraged Bill to the point of growing into his ‘angry form’ and surrounding the room in flames, cornering the protagonists atop a giant stone Stan, where he tortured them by making them experience their worst nightmares. After killing Mabel’s two dream characters and threatening to do the same to Mabel and Soos, Dipper arrived just in time, having been taught by Stan in the personal memory that they can do whatever they can think in the Mindscape. They then used this to fight and overpower Bill.

Before they could defeat him though, Bill teleported them all to a blank white space, proving he was still far more powerful. But instead of killing them he stated he was impressed by them, letting them live since they might be useful later. He gave them a warning that a darkness was approaching and that everything they cared about would change. He promptly teleported away, saying he’d be watching them until then.

Season 2
Bill later returned in the Season 2 episode Sock Opera, where he offered to help Dipper find the password to The Author’s laptop in return for a ‘favor’. Dipper denied his offer and Bill disappeared, telling him he’d be back when he was ready to change his mind. He later reappeared when Dipper failed too many times to guess the password, forcing the computer to erase all its data after a countdown. During the countdown, Bill reappeared, stating Dipper looked desperate and reprised his offer, taking advantage of Dipper’s desperation and telling him that he only wanted a puppet. Dipper tried to resist, saying the puppets were for Mabel’s sock puppet play, but Bill was able to convince him by asking him what she had done for him lately, reminding Dipper of how much he’s sacrificed for her without her doing anything in return.

Dipper finally agreed to the deal, only for Bill to remove his soul from his body and possess it, making Dipper the ‘puppet’. He then smashed the laptop and proceeded to express his joy and ‘test’ his new body by slapping it in the face. Upon being asked why he had broken his word, Bill revealed that Dipper was getting too close to major secrets and that he didn’t need the boy getting in his way. He then left to destroy Journal #3 to stop him for good. After Mabel unknowingly told ‘Bipper’ she was using the journal as a prop in her play, he convinced her to let him play a part in it in order to get closer to the book. Dipper followed him there though and was able to tell Mabel what happened, and she went to get the book.

However she was confronted by Bipper, who threatened to drop Mabel from the catwalk and ruin her play if she didn’t hand over the journal. Mabel almost gives him the journal and he asks her who would sacrifice everything they’ve worked towards for their sibling. Mabel responded by telling him Dipper sacrificed for her, and returns it by throwing the play. Mabel and Bipper had then fought onstage, with him easily overpowering her. However, she exploits Dippers body’s weaknesses, such as the fact that he hadn’t slept in more than 50 or so hours and is able to defeat him. With the body exhausted to the point of unconsciousness, Bill is thrown out of Dipper's body and Dipper takes it back. The twins share a moment, but Bill possesses one of the sock puppets, telling them they can’t stop him. Mabel sets off the fireworks used in her play, destroying the puppet and forcing him once more into retreat.

Bill appeared a third time for a short cameo at the end of Northwest Mansion Mystery. After, Old Man McGucket fixed the laptop and warned Dipper that the end of the world was coming and that they had to stop it, as the screen panned off to a tapestry with a black and red Bill on it gazing over a burning landscape, alluding that Bill was what was to cause the apocalypse.

While not specifically present, Bill was clearly referenced numerous times in the episode A Tale of Two Stans, first when a younger McGucket spoke the ominous phrase "When Gravity Falls and earth becomes sky, beware the beast with just one eye", and again when Ford, Stan's brother, inspected his shape out of suspicion of him being possessed by Bill, notably by checking the give away eye shape with a flashlight.

He then made another direct, though off-screen appearance at the end of the episode "The Stanchurian Candidate", when Gideon attempted re-summoning him from his prison cell to strike another deal, possibly to destroy the pines family in exchange for a host to possess in Gideon.

Bill finally made his third appearance in the episode "The Last Mabelcorn", where he came to Ford in a dream, establishing their past relationship and warning Stanford that he was coming. In response, Stanford had Mabel retrieve Unicorn Hair in order to cast a spell that would protect the Mystery Shack from Bill Cipher. In the meantime, Stanford showed Dipper a machine that would scan their thoughts and shield their minds from the demon as a last resort in case Mabel failed. Curious of Ford's thoughts and his history with Bill, Dipper attached the machine to Ford in his sleep and saw that he was dreaming of Bill and Ford making a deal. In his horror, Dipper also knocked over the curtains on the walls, revealing the room to have once been a shrine to Bill as many paintings of him hang from the walls and statues of him stand in the corners. Because of this Dipper believes Ford to be possessed by Bill, or worse, in league with him, to the point of almost using the Memory Eraser on him. Ford proves he's still on their side though and reveals his history with Bill. Apparently, Stanford hit a roadblock in his research of Gravity Falls, but found writing in a cave depicting a being of knowledge that could help him, leading him to summon him.

Bill first appeared to Stanford in a dream, where he seemed to be affable and friendly, offering a game of chess and "Dream Tea". Bill lied to Ford, telling him he was a muse that chose a brilliant mind to inspire every century. In his flattery Ford made a deal with Bill, that in exchange for total access to Ford's mind Bill would help Ford by telling him to build the portal. But when McGucket was sucked into the portal, Bill's true plans were revealed and the triangle exposed as a malevolent, psychotic monster. It is learned he was going to use the portal to open a gateway between the Nightmare Realm he exists in and the physical world, allowing him to achieve physical form and unleash his fellow demons, in his own words "teach this dimension how to party". It is also revealed that Bill is older than "anything in our universe", and that Bill had been manipulating history by making deals with people throughout time, apparently for the sole purpose of one day creating the portal. Ford tells Dipper this is why they must protect the Dimensional Rift, and that if they worked together they could be the last people to be tricked by Bill. After the spell is successful, Ford tells Dipper that anyone inside the bubble surrounding the shack would be safe from Bill's influence. However, Bill is watching them from his dimension, and decides that if he can't affect anyone in the Mystery Shack, he would have to look for his next pawn on the outside as a series of images of the rest of the cast flash over his eye.

Bill next appeared in "Dipper and Mabel VS The Future", where he possessed Blendin Blandin in order to have a vessel to destroy the rift. Using Blendin, he convinced Mabel to steal the rift for him by promising her that he can extend summer for her with it for as long as she wanted. As it turns out, the rift is in Dipper's backpack (which Mabel took by accident). Upon receiving the rift from Mabel, Bill promptly smashes it on the ground, opening the gateway to the nightmare realm. Leaving Blendin's body, the episode ends with Bill laughing triumphantly at Ford and Dipper as he makes his entrance into the world and ignites "Weirdmageddon".

Weirdmageddon
In "Weirdmageddon - Part 1", Bill finally attains a physical form and frees his gang from his dimension. He then wreaks havoc on the town of Gravity Falls and turns it into an apocalyptic, nonsensical wasteland while declaring that he intends to do this to the entire universe. Dipper and Ford then enact a plan to take out Bill with a quantum destabilizer, but fail due to the abnormal environment. After being captured by Bill, Ford is offered the chance to join him, but is turned into a golden backscratcher by the demon after refusing. Dipper tries to stop Bill, but his lack of information of the dream demon causes him to be easily defeated. Bill then proceeds to burn all three journals and mocks Dipper of his effort of being a hero, saying that those who try to become heroes always end up destroyed by Bill himself. Bill then sicks his henchmen, 8-Ball and Teeth, on the boy to eat him and flies up to his new, floating castle to party with his crew. After three days of non-stop partying, Bill is confronted by Time Baby and his police force, who came to arrest him for both using the body of Blendin Blandin for his scheme and to stop his plans for good. Unfortunately, Bill is unfazed and promptly kills Time Baby and his police squad by disintegrating them, much to the shock of Blendin, who escapes away.

Meanwhile, Dipper, after escaping from 8-Ball and Teeth, is on the search for his sister, Mabel, who Bill imprisoned in order to honor his deal with Gideon (who he also broke out of jail). After a confrontation with Dipper and Wendy, Gideon calls upon Bill's minions to take them and Soos to the dungeon in Bill's castle. After being convinced by Dipper that Mabel will never love him and that he can't force her to, a newly reformed Gideon decides to help aid in his former enemy's cause by giving him the key to Mabel's imprisonment bubble. He and his prison goons then leave to confront Bill and his minions.

In "Weirdmageddon 2: Escape From Reality", Bill have captured much of Gravity Falls' residents and petrified them to form his own throne of human agony for his enjoyment. He taps a fork on Golden Ford to grab the partying monsters' attention, and announces that they will now unleash their chaos on the whole world. As all other monsters take to the skies to spread destruction, they slam into an invisible forcefield without warning, much to Bill's complete anger. He figures out that their chaos has formed a barrier around Gravity Falls, the only area they can currently reside in, meaning that they can't spread their destruction throughout the world. Upon learning of this, Bill realizes that this might be more difficult that he thought. One monster complains that he broke something, and Bill angrily retorts, "WALK IT OFF!"

Upset by this turn of events, Bill considers getting Ford 'out of retirement', believing that the latter may have the answer to breaking down the barrier. He then finds out from Keyhole that Gideon and his goons let Dipper, Wendy, and Soos into Mabel's bubble. Bill simply laughs, explaining how the bubble is his most diabolical trap of all. He tells Keyhole to fetch Gideon and his goons as he prepares to expand his world upon Earth, and later, the universe. After Gideon and his goons are caught, Bill proceeds to punish Gideon by locking him up in a bird cage and forcing him to cute dancing non-stop in sailor's suit while the prison goons end up being petrified and added into Bill's throne, much to Gideon's dismay.

In "Weirdmageddon 3: Take Back the Falls", Bill releases Ford from his frozen petrified state to gain knowledge about the forcefield that's keeping him in Gravity Falls. Ford admits that he knows of a certain equation that will collapse the barrier, but angrily refuses to tell it, knowing that it would result the end of the world. Bill establishes that he comes from a flat dimension where he is implied to have destroyed his home diminsion, including his family (hinted back in the 2015 Bill Cipher AMA on Reddit). Ford refuses to cooperate with him, which instantly tests Bill's patience. Bill starts torturing Ford in order to get the answer to his dilemma, though Ford stands his ground despite the painful torture. As soon as the Pines family and their resistance group made a giant robot to fight their way to the Fearamid, Bill orders his henchmaniacs to fight the resistance, but they fail as the townspeople worked together to take down most of the henchmaniacs. As Ford compliments Dipper and Mabel for a job well done, Bill finally decides to torture both kids in order to coerce Ford into giving him the equation to collapse the barrier. He takes matters into his own hands and tries to destroy the robot, but with no success, due to the Mystery Shack being protected by unicorn hair that Dipper and Ford enforced back in The Last Mabelcorn. The robot tears out Bill's eye, where he complains to the resistance on how long it will take for it to regenerate, which allows the twins and several citizens to arrive to the Fearamid and release all the petrified citizens (including Ford, Gideon, and the prison goons). Unfortunately, as soon as Bill regenerates a new eye, he instantly finds the robot's weak spot and uses it to take down the robot easily.

The Pines family unites with some of the other townspeople in order to learn about Bill's weakness. They learn about the Bill Cipher Wheel and tried to use in order to defeat Bill for good, but it fails, as Stan refuses to hold hands with Ford due to bearing a grudge and holding some resentment for his twin brother over the whole portal incident. This allows Bill and his remaining henchmaniacs to finally gang up on against the townspeople. After trapping Ford and Stan, Bill transforms Soos, Wendy, McGucket, Pacifica, Robbie, and Gideon into tapestries in his lair when they try to gang up on him. Dipper and Mabel risk their lives by angering Bill with their pranks in order to distract him, and he transforms into a horrifying multi-tiered being in his rage to hunt them down once and for all, while Stan and Ford try to figure an alternative solution.

Death
When Bill finally captures Dipper and Mabel, he ponders on who to kill first, which finally forced a desperate Ford to surrender himself and proposes a deal: he will let Bill get access to his mind in return for letting the twins and his brother go free. Bill gladly accepts the deal and instantly leaves his physical form to enter Ford's mind. However, it turns out Ford and Stan planned a switcheroo and Bill ends up in Stan's mind, where Stan taunts him for falling for the trick. Ford, with no other option, reluctantly uses the Blind Eye Society's mind-erasing weapon to wipe out the memories of his twin brother in order to defeat Bill for good. Realizing that he's been tricked, Bill angrily calls off the deal, but he gets locked within Stan's mind as the dreamscape and living room of the Mystery Shack burn in flames thanks to the mind-erasing weapon. A terrified Bill pleads for mercy by appealing to Stan's greedy nature with riches and power, but Stan angrily refuses and proclaims that Bill has harmed the Pines family too much to deserve any mercy. As Bill begins to glitch, he furiously cries out at Stan and rushes at him, but Stan retaliates by punching Bill directly in the eye, obliterating the screaming dream demon as Bill shatters into pieces, erasing him from existence and ending his reign of terror just before Stan's memories are completely wiped out.

Right after, everyone that Bill transformed into tapestries are free as Weirdmageddon immediately soon winds to an end when both the Henchmaniacs and the chaos are forcibly sucked back into the Nightmare Realm, from where they had came. The entire valley and town of Gravity Falls is then restored as everything returns back to the way it was before the whole tragedy even began. Shortly after, the only thing left of the event was Bill’s petrified body that was seen lying isolated somewhere deep within the woods and is now one with nature itself.

Foreshadowed Return
Although erased from existence, there are possible hints that Bill could somehow possibly return or resurrect; while interrogating Ford at the Fearamid, he sings “We Will Meet Again”. Right after the closing credits, the lifeless statue of Bill is seen for a brief moment in real-life as part of the new game, Cipher Hunt and in a non-canonical book titled "Dipper and Mabel and the Curse of the Time Pirates' Treasure!", where the Twins go through time, one enormous thing that It is speculated that the canon secret could be Dipper and Mabel's encounter with the Axolotl in "the time and space between time and space”, a giant talking axolotl, lets each of them ask him a question. Dipper asks him "what [does he] know about Bill Cipher” to which he replies with a poem: ''Sixty degrees that come in threes. Watches from within birch trees. Saw his own dimension burn. Misses home and can't return. Says he's happy. He's a liar. Blame the arson for the fire. If he wants to shirk the blame, He'll have to invoke my name. One way to absolve his crime. A different form, a different time''.

This suggests that when, before being erased from reality in the series finale, Bill gives a message that when reversed says "A.X.O.L.O.T.L.! My time has come to burn! I invoke the ancient power that I may return!", he is actually invoking the Axolotl for redemption for his crime, and that there is one way to get absolved: he has to take "a different form" in "a different time". This all implied the possibilities that Bill may get resurrected after another series of generations to come.

Aftermath
Following his demise, Ford and the Twins soon find Stan out in the open, but sadly, he has no idea who they are, Ford then explains to them that Stan bravely sacrifice himself to defeat Bill, in order too save them and everyone else. However with the help from both Dipper and Mabel Pines, they were able to restore their loving uncle as he slowly remembers more and more about himself and his family. Sometime after the event, things have fully returned to normal in Gravity Falls Oregon, Stan had already recovered from his amnesia and the Pines family were declared heroes.