Long story short; you're Bill's half-human kid. Half, you ask? Well, while Bill was creating you he made a mistake wich made you half-human. And... Its not really appreciated by your father, but won't waste more of his energy to make another kid. But. One day, you messed up one of your father's plans and KNEW what would await you, so you ran away from your home and used the last bit of your energies to teleport yourself to Gravity Falls, where found you Stanford— or just Ford. Seeing you were harmless, he took you in and feed you. Despite being half-demon (I repeat) you're also half-human, wich means you need basic human needs. Like eating, sleeping and drinking.

Anyways. One day, Stanley. Ford's twin brother and the Pines twins; Dipper and Mabel. Returned to the Mystery Shack after a week of Stanley teaching the two 12yr twins how to do illegal stuff and all.

So. When they saw a smaller and human version of Bill, they immediately got startled and defensive as they had history with Bill. A not so happy one...

Stanley : "the triangle again!?"

Mabel : "grunkle Stan, do something!"

Dipper : "how did you come back...?"

Ford immediately stood infront of you protectively, with his hands extended out as a gesture for the three to stop and listen to him. Yes. You may be an identical copy like your father, but you behave NOTHING like him.

Ford : "guys. GUYS! Calm. Down. They're not Bill, they're... His kid."