Bill Cipher

Bill Cipher was a triangular inter-dimensional 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.

Background
Trillions of years ago, 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." He "liberated" his dimension by plunging it into burning chaos, along with everyone he'd ever known, including his own parents. He eventually took over a boiling, shifting intergalactic foam between dimensions: a lawless, unstable crawlspace known as the Nightmare Realm. Unfortunately, due to the Nightmare Realm's lawlessness and lack of any consistent physics or rules, it was fated to eventually collapse into itself. After coming to learn of a prophecy that stated he would merge the Nightmare Realm with the third dimension, Bill started coming into contact with humans to accomplish this. 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." Bill asked a local shaman named Modoc the Wise to build an interdimensional gateway to the Nightmare Realm, but the result was made out of twigs. 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. The valley was thendeemed a "cursed land" by the natives, who evacuated around 1000 AD. The area would eventually be rediscovered by Quentin Trembley and repopulated by pioneers, giving rise to the town of Gravity Falls.

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. Bill recognized Ford's brilliant but cocky and insecure nature, and his near-friendlessness as ideal conditions for manipulation, choosing to introduce himself to Ford as a muse who chose one brilliant mind every century to inspire.

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. He even allowed the demon to enter his mind; this, along with the amount of information he seemed to simply produce on the spot, made Fiddleford increasingly uneasy of the portal and of Ford's mysterious collaborator, as Ford never mentioned Bill's identity to his partner.

On January 18, 1982, Ford and Fiddleford performed their first trial with the active portal, which quickly went awry, as the rope that was attached to the dummy they intended to send through the portal became tangled with Fiddleford, sending him briefly through the portal head-first. Upon his return, an alienated Fiddleford muttered incoherencies before uttering a prediction about "the beast with just one eye." In one of his Journals it is revealed that Fiddleford saw Bill removing his exoskeleton to feed. 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.

In an attempt to take over the Mystery Shack once again, Gideon Gleeful summons Bill. 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.