Sigil, or commonly known as the Shape-shifter is a creature seen in "Joshua and Margaret Investigations". It is a shapeshifting being apparently from an alien world or another dimension that infected Joshua with its venom, causing him to develop a large boil on his head from which Jake emerged. It is, technically, the biological "mother" of Jake, though Jake does not know this. Tree Trunks mentioned to Joshua and Margaret that the creature had touched her and several other citizens of Ooo on the shoulder, and it cackled knowingly before opening a portal and returning to its home world, suggesting that it might have been seeking to create some sort of hybrid offspring on this world, the result of which was Jake. It resided in the Pantheon Of Savings, or at least when it was not in its home world.
The creature is midnight blue in color, has four legs, and has five separate yellow eyes with red pupils in the order of a star. The eyes change in number with each transformation as well as the shape of its body and legs. It possesses a circular mouth split in a varying amount of directions and many sharp teeth. It has the ability to change into what appears to be an owl, a cat, a baby and many other unknown forms.
It can manipulate its body into any shape or size and has stretchy powers like Jake but uses them in a much more fluid fashion. It can shapeshift into multiple creatures like a baby, humanoids, a cat, owl and many other unknown forms. The creature can also change into a glyph-like shape and shoot beams of energy from it's many eye stalks to open a portal to another dimension which resembles a dark blue lava lamp, and it's because of this that the creature has been named Sigil. It can also reproduce by impregnating other organisms by biting them if the creature deems them compatible (in Joshua's case, he was bitten on the head).
- Jake Jr.'s mouth is similar to the creature's mouth, meaning that it is a gene passed down from the creature, to Jake, then to Jake Jr.