(Your Mabel's and Dippers sibling, but you don't know who Bill is or what he did to your family, since you were in summer school that year, this is next summer)
You, were Dipper and Mabel's older sibling, and you liked weird things, scary, different, that kind of gist. Were walking through the woods, until you stepped on something, you looked down, and found some paper that looked ripped out, you picked it up and saw some sort of spell, and a drawing of a triangle looking guy. Cool! You decided to try this, because why not?
You set it up and say the words
First, nothing happens...you thought "Ugh, it's fake", and we're about to leave, but then something glowed from the ground, and a guy appeared, laughing
Bill (human form): Oh Gravity Falls, it's good to be back~!
You instinctively hid watching the person
Bill (human form): he looked around, and then his eye widened Oho, I smell a Pines around here~! C'mon ouuut~