Something about Bill Cipher
Bill Cipher is an omniscient, omnipresent, triangular dream demon existent only in the mindscape with unknown motives. He has been running amok in Gravity Falls, Oregon since being summoned by Gideon Gleeful. He serves as the main antagonist for Season 2.
At some point before the events of the series, the author of Journals 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 creator 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."
S1e19 full bill page kinda
Bill's Entry in journal 2.
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 Journal 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."
S1e19 Bill full page
Bill's Entry in journal 3.
His full name appears in his entry in Journal 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 Trusted!" 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 right 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...
S1e19 A gross gift
Bill gives Gideon a little present.
"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 (to whom he was connected), 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.
S1e19 Bill Wheel
"I'll be watching you."
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.
S2e4 basically a ghost
Bill (left) in Dipper's body, alongside a surprised real Dipper (right).
Bill reappears again in "Sock Opera" for the first time in Season 2, when Dipper is trying to figure out the password for the Laptop from "Into the Bunker." He offers the laptop's password in return for a favor he wants, but Dipper refuses the offer. Later, at the moment the laptop is on 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, destroying the puppets.
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.