Bill is the eye of providence, aka the all-seeing eye. He is seen on the $1 and $-12 bills. He's a dream demon, who knows lots of things but not everything.
Seen everywhere
Bill is seen in numerous places in Gravity Falls well before his official introduction. I won't list them all off, but the question is: why? Why would the buildings and items in Gravity Falls have Bill's face on them?
goat
There's a theory that Bill is Gompers the goat. The first evidence comes from Tourist Trapped, when Gompers is standing on Dipper's bed. This raises the question of why he was indoors in the first place. If he's Bill, though, that explains it.
If Bill is Gompers, it would explain why he tried to take Stan's fez at the end of the Stan Wrong Song. The fez probably has something to do with the ROotHM. The ROotHM is probably against Bill. So Bill stole Stan's fez. I believe the other members of the ROotHM replaced it, but at the cost of a demotion, accounting for the change in symbol.
Bill is watching them. I won't list off all the examples, but one that comes to mind is when Gompers watches Stan, Soos, Dipper and Mabel fall into the Bottomless Pit.
The goat makes another appearance in the attic, after Mabel's sleepover in Carpet Diem. Mabel, Candy and Grenda had just had a wild sleepover, but apparently can't remember all of it. I think Bill snuck into their minds and erased a few things.
Lots of things...
When Bill says this line, he flashes pictures. They include images from other episodes of Gravity Falls:
- the statue of Nathaniel Northwest
- the cave room where Quentin Trembley's body used to be
- Stan entering his secret lab
- the Gideon-bot's broken arm
- and the Gravity Falls prison,
as well as famous conspiracies:
- JFK's assassination
- UFO sightings
- the Aztec calendar
- the Apollo moon landing
- Stonehenge
- the pyramids of Giza
- and crop circles.
All the conspiracies mentioned above (except the JFK assassination) have something to do with space and/or extraterrestrials.
darkness
Bill says that "a darkness approaches. A day will come in the future when everything you care about will change." Could this darkness be related to his earlier line that "reality is an illusion the universe is a hologram..."? Maybe everyone finds this out, and this is how everything they care about changes? Could he simply be referring to Gideon's imminent takeover of Gravity Falls? It's pretty creepy, and I'll have to admit that I don't like Bill because of his creepiness.
Bill left one final message in the credits of Sock Opera: "No puppet strings can hold me down, so patiently I watch this town. Abnormal soon will be the norm, enjoy the calm before the storm." It seems to me that part of the prophecy came true in Society of the Blind Eye, as abnormal is now the norm. The rest will come true (I think) in Not What He Seems. The calm is over, and it's going to be one heck of a storm.
main season 2 antagonist?
Thanks to Alex Hirsch's AMA, we know for a fact that season 2 will include Bill Cipher. My personal theory is that he will be the season's main antagonist, and that eventually, Gravity Falls will come down to some kind of all-out battle: Bill vs everyone on his wheel.
power
After a lot of pondering, I decided that Bill has almost as much power in the mindscape as he does in the mind, i.e. he can do pretty much anything he wants. The only stipulation is that he can't affect a person without making a deal of some kind. We still don't know what he wants, but now we have a clearer idea.
There is a cryptogram which probably describes Bill in Sock Opera: "Pure energy, not skin and bone, rising like the Shepard tone." Bill says later in the episode that he's "a being of pure energy with no weaknesses!" That's what he wants you to think. But, the Shepard tone is a sound that seems to be perpetually rising in pitch, but is really just an illusion. Same with Bill. He seems to have no weaknesses, but it's an illusion. I would say that people have just as much power as he does, but they don't know it.
