Bill Cipher is an inhabitant of a two-dimensional universe called Euclydia. He doesn't appear in the Gravity Falls animated series very often, but is frequently hidden in small details that only the attentive viewer can see. Originally existent only in the Mindscape, later Bill was able to get into the real world in a physical form that featured a striking yellow triangle with limbs and one eye, that allowed him to see the third dimension. Bill is known as a dream demon who despised living in his world, describing that place as a dimension of "flat minds in a flat world with flat dreams". A fanart Gravity Falls cursor featuring the mysterious Bill Cipher.
