Bill Cipher

Let's talk about a very important character in Gravity Falls. Bill Cipher is a dream demon.

Source: gravityfalls.wikia.com

He can enter a human's mind and he can see everything that anybody does . You might be wandering why he crucial in the show. Well, let me explain. If you look around the Mystery Shack you can notice there are lots of triangles with a eye in the middle. Sounds familiar? No? Well, it should. Look at Bill. So now, if you understood that part, let me tell you something. In every single episode of Gravity Falls, he appears in some sort of way. On a rug, on the -12 dollar bill (negative $12 bill) seen in the episode "Irrational Treasure" (season 1 episode 8). In the episode "A Tale Of Two Stans", McGucket warns Stanford about Bill. So we know that he is dangerous, seeing that he creates chaos, that probably means he is a threat.

Let's get to the episodes that actually show of how powerful and terrifying Bill really is. The episode "Dreamscaperers" is one of the first times that we are introduced to Bill. Bill was summoned by Gideon. Gideon and Bill make a deal: if Bill helps Gideon getting the code to the safe (the safe has the dead for the Mystery Shack locked inside) out of Stanley mind, Gideon would do Bill a favour. So Bill enters Stan's mind to steal the code. That is the place where Dipper, Mabel and Soos fight Bill. In the end, our heroes almost defeat Bill, but Bill stops the battle and warns them that in the future there will be a time when everything that they know is going to change. Then the wheel seen at the end of the theme song appears around him and Bill disappears. Just like this:

The second episode where we see him is "Sock Opera". In this occasion, he enters Dipper's body and uses Dipper as his puppet. Eventually, Mabel defeats Bill and Dipper gets back into his own body. But entering minds and taking control of someone's body is not all he can do. Here's a list with all his abilities, as recoded by gravityfalls.wikia.com:

Telepportation
Clairvoyance
Cross-dimensional awareness
Illusion Manipulation
Intangibility
Innate capability
Laser manipulation
Levitation
Molecular Manipulation
Nightmare Inducement
Possession
Precognition (of potential scenarios)
Pyrokinesis
Size shifting
Telekinesis
Telepathy

In the trailer at Comic Con 2015, you could see this picture:

If one remembers the circles left by aliens in a crop, well, this exactly how Bill looks in the picture. My guess is that we can expect that Bill will restart producing chaos near the end of this season. Bill's catchphrase leads to the same idea: "Remember, reality is an illusion, the universe is a hologram, buy gold, bye!"