"(Laughs; Deep Voiced) Alright listen up you, one lifespan, three-dimensional, five sense skin puppets! (Normal Voiced) For one trillion years I've been trapped in my own decaying dimension, waiting for a new universe to my own. Name's Bill! But you can call me your new lord and master for all eternity!" -Bill Cipher
Bill Cipher is the main antagonist in Gravity Falls. He is a Dream-Demon with mysterious motives, and seems to have a vendetta against the Pines family, especially Stanford Pines. And he was defeated by Stanley Pines, when Stanford erases his mind.
He can be seen during the final battle.