The Shape Shifter, (a.k.a. Experiment #210) is a creature able to take on any form that it has seen.
The two initially planned to use the creature to test their underground laboratory’s cryogenic stasis chambers, but Ford was fascinated by his ability to change forms and learn speech, and kept the Shape Shifter in a cage to study, much to his nervous assistant’s displeasure. Despite being fond of the Shape Shifter and thinking of him as a pet, Ford always wore a surgical mask while interacting with him and refused to show him any of the monsters contained within his journal as a precaution against any trouble he might cause.
These precautions eventually angered the Shape Shifter, who decided to take McGucket’s form and trick Ford into giving him his journal so he could gain greater power, a trick which almost worked until Ford discovered a tied-up McGucket next to the Shape Shifter’s broken cage. The two then outsmarted the creature by leaving a decoy journal in one of the cryogenic tubes and freezing him when he came to collect it.
Afterwards, Ford and McGucket agreed to seal the entire bunker off due to the unpleasantness of the experience. While they were unaware of this at the time, this action later prevented the Shape Shifter from leaving the bunker once he broke free of his stasis tube, keeping him trapped below ground until the present day.
When Dipper and Wendy first met the Shape Shifter, it was in the form of a bean can mascot, pretending to be the author of the journals in order to steal Journal #3 from Dipper, who mistakenly gives the Shape Shifter his Journal. After looking through it, the Shape Shifter is fascinated by the many new forms it is able to take. After realizing the danger they are in, Dipper tries to take back the journal. Wendy is then able to stun the Shape Shifter so that it drops the journal, and the two flee.
After running into Mabel and Soos, they formulate a plan to get rid of the Shape Shifter. Dipper, Mabel, and Soos blast a jet of water at the creature, accidentally hitting Wendy. The jet temporarily stuns the Shape Shifter, but it attempts to trick Dipper by taking Wendy's form in one last effort to steal the journal. The Shape Shifter attacks Wendy, and Dipper becomes confused by who is who, asking the real Wendy to give him a sign. When the real Wendy zips her lips, Dipper hits the Shape Shifter with Wendy's axe, and the two of them push the monster into the stasis tube to freeze it. Before becoming completely frozen, the Shape Shifter gives Dipper one last warning about prying too deeply into the mysteries of the town and the author, saying it will end with a fate that is worse than he could imagine. The last form it takes is Dipper screaming, saying it is "the last form [Dipper] will ever take" (an event that later happens in "Northwest Mansion Mystery", but is reversed in the same episode).
It also appears in Ford's flashback in "A Tale of Two Stans" as a recently hatched specimen which took on the shape of the mug that was beside it when it hatched.
The Shape Shifter is a cunning, quick-thinking and manipulative creature. Without even knowing Dipper, Wendy, or their quest for the author, it was able to effectively set up an elaborate ruse to win their trust and gain the journal.
The Shape Shifter's default form is a pure white slimy body with four slender legs. Its right arm is slender and has a three-fingered hand, and its left arm is swollen with a clawed hand. It has two baby pink eyes and a snout with four fangs hanging on the outside of its mouth, and six smaller teeth on the inside.
Its abilities enable it to change not only its appearance but also its entire form, including its voice. Thus, it can be as small as a Gnome or as tall as the Hide Behind or as amorphous as fire. Its desire for the journals in order to assume different forms seemingly indicates that it can't change into new forms unless it has "scanned" a form or has seen a picture of it. It can also "mix" forms to create new monsters, or become two things at once.
- A note in the surveillance room says - EXPERIMENT #210 - "THE SHAPE SHIFTER" BECAME TOO DANGEROUS! PLACED IN CRYOGENIC STASIS.
- The Shape Shifter has been shown to turn into a man based on the mascot of a brand of canned beans, a Gremloblin, a gnome, the Hide Behind, Mabel, Dipper, a Dipper-Mabel composite monster, Wendy and six currently unknown creatures: a spider creature (shadow only), a three-eyed toad, a creature with a fist for a head, an insectoid with an armadillo shell, a golem-like creature, and a creature that resembles some burning fire in "Into the Bunker." It can also turn into a cup similar to the one beside it when it hatched from an egg in Ford's flashback in "A Tale of Two Stans."
- The Shape Shifter can transform into the Hide Behind, despite the fact that Journal 3 has no clear photo of it.
- Its true form was also inspired by Clayface from the animated Batman series and is based off of a Vietnamese eggroll that character designer Robertryan Cory once ate.
- Another reference to the animated Batman series is that it shares a similar, if not identical, roar as Man-Bat from the episode "On Leather Wings."
- According to Gravity Falls: Journal 3, The Shape Shifter's DNA constantly changes, probably due to the many forms it takes.
- The cryptogram on the Shape Shifter's entry in 3 reads: "AM I ME?, IS HE ME?"
- It appears on the Gravity Falls Season 2 poster.
- In "Dipper and Mabel vs. the Future," it is possible that the Shape Shifter (or an ancestor of it) is mentioned in the cryptogram found in the UFO, which, once decoded, reads: "SPECIMEN HAS ESCAPED, IS CHANGING FORMS."
- The white, slimy form the Shape Shifter is first seen in may not be its true form, as the illustration in the journal is accompanied by a caption reading "Form #6."
- Although this could just be the sixth form Ford saw it in.
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