Bill Cipher is a dream demon, who has appeared in episodes as pictures, Easter eggs, and even himself, he is also evil.
Mirra often complains about and blames Bill for things that go wrong. He has appeared to her in dreams and sometimes appears to her when she closes her eyes.
Bill is one of the most mysterious of the creatures of Gravity Falls. Bill is an insane and physically irreverent demon who finds most things amusing (although no one is really sure why). He's outrageous and outlandish. Though he may come across as simply annoying, he's not to be taken lightly; when angered, he's a force to be reckoned with. The reason why his name is Bill is because he is on the dollar bill.
2. Sock Opera
3. Unnamed 12th Episode (Possibly?)