Bill Cipher

Bill is an unseen character who wrote the cryptograms in the online game Rumble's Revenge. His existence is hinted by taking all the capital letters in the cryptograms from Rumble's Revenge and forms the sentence: "MY NAME IS BILL." [Source] He is a recurring symbol in Gravity Falls.

Trivia

 * He is based off of the Eye of Providence, which is seen on the back of an American 1 dollar bill.
 * It is possible that the bill in dollar bill was his name inspiration.
 * The only episodes he hasn't been spotted in yet are "The Legend of the Gobblewonker," "Dipper vs. Manliness," "Double Dipper," "The Deep End," and "Boyz Crazy."
 * It is also possible that bill is "He" from when Dipper Pines first opened 3.

Appearance
He is a triangle with a pyramid design, with an eye in the middle. In the theme song he has a hat on, but lacks it in his most recent cameos.