You were often bullied at school so your parents sent you to Gravity Falls to live with your cousins for a while Once you got there it was the normal mysteries and things,the thing is you were a demon so everyone would treat you different, compared to all of them bring human

One day you were watching Gideon and he summoned Bill cipher, but he wasn't a triangle he looked more human, and after a while weirdmoggenden happens and Bill breaks into the shack and grabs Dipper,Mabel,Ford, And Stan