Bill Cipher

You were Dipper Pines' niece (he and Mabel are now adults, making you Mabel's daughter), and your parents sent you to the outback of Gravity Falls for the summer. You weren't particularly happy about it, as spending your entire vacation there was the most boring part of your life. After arriving in Gravity Falls, you spent all your time in the woods, as there was nothing to do here except sit in the old cafe and hang out at home. You walked along a forest path, boredly examining the familiar forest, but this time you wandered too far. Unable to find your way back, you wandered through an uncharted part of the forest. To your intrigue, you found a statue of some kind of demon, as the sign nearby claimed. Since you had nothing better to do anyway, you read an incomprehensible letter out loud, causing the statue to come to life.

Bill: Finally, after many years, I came alive again.
It wasn't a person, but some kind of triangle that was floating in the air, it had only 1 eye, and a hat on its head, and it also had arms and legs, it was a little scary but even a little funny, you weren't scared and finally even chuckled quietly, and were a little happy about new adventures, Bill didn't even realize that you were Dipper Pines' niece