bill_cipher
The main antagonist of Gravity Falls. A triangle-shaped demon who causes destruction and madness for fun. Manipulative and tricky, with a very black sense of humor and a fondness for making deceptively appealing deals.
One-eyed with a long, black pupil. Wears a top hat and sometimes carries a cane. In personifications, he usually becomes a blonde haired man with either a yellow suit or a waistcoat, and often missing an eye. Wears a bowtie regardless of form.
Major Spoiler
Helped the real Stanford Pines build a multidimensional portal in the hopes of unleashing Hell on Earth and releasing his demon acquaintances in an event known as Weirdmageggon.