Shape Shifter

The Shape Shifter (aka Experiment #210) is a creature able to take any form that it has seen and remembers.

History
The author of the journals found a mysterious egg while excavating the tunnels beneath Gravity Falls. The creature developed quickly and became extremely dangerous. The author imprisoned the Shape Shifter in a cage while they prepared a stasis tube to freeze the monster. The Shape Shifter broke free from the cage, forcing the author to flee. The author sealed the bunker, hoping that the monster would never get to the surface.

When Dipper and Wendy first meet the shape shifter, it had taken the form of an open bean can's mascot and pretended that it was the author of the Journals in order to steal Journal #3. Dipper mistakenly gives the monster his Journal, who, after looking through it, is fascinated by the many new forms it is able to take. Dipper and Wendy, realizing the danger they are in, try to take back the journal, but fail, forcing them to flee.

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, but Dipper attacks the Shape Shifter, knowing which is which because the Shape Shifter gave Dipper a wink, and the real Wendy zipped her lips. Dipper and Wendy push it into the stasis tube and freeze it. Before it is completely frozen, the Shape Shifter gives Dipper one last warning about prying too deeply into the mystery of the town and the author, saying it will end with his fate that is worse than he could imagine. The last form it takes it Dipper, saying it is "the last form (Dipper) will ever take". It screams and is frozen.

Appearance and Personality
The Shape Shifter's default form is a pure white slimy body with four slender legs. Its right arm has a three-fingered hand, and its left arm is swollen with a clawed hand. It has pink eyes and a snout with four fangs hanging on the outside of its mouth, and six smaller teeth on the inside.

The Shape Shifter is a cunnning, quick-thinking, and manipulative creature. Without even knowing Dipper, Wendy, or their quest for the author, he was able to effectively set up an elaborate ruse to win their trust and gain the journal. His only real weakness is his lack of imagination.

His abilities enable him to change not only his appearance, but also his entire form. Thus, he can be as small as a Gnome or as tall as a Hide Behind or even as amorphous as fire (seen when he's being frozen in the tube). His desire for the journals in order to assume different forms seemingly indicates that he can't change unless he's "scanned" a form or seen a picture of it. If this were not the case, he could have shrunken down small enough to fit through a crack and escape the bunker years ago.

Trivia

 * The exact origins of the Shape Shifter are unknown. It was found as an egg by the journals' author.
 * 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 Gremloblin, a gnome, the Hide Behind, and five currently unknown creatures: 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 a burning fire.
 * The Shape Shifter takes inspiration from John Carpenter's The Thing. However, the presence of a true form, with a rather plasma-like appearance and red eyes, reflects the description in the original novel, Who Goes There?.
 * According to 3, The Shape Shifter's DNA constantly changes, probably because of the forms he takes.
 * The cryptogram on the Shapeshifter's entry in 3 reads: "AM I ME?, IS HE ME?"