*In one sub-space of the original world called the CodeSphere, the AUs and ATs exist with its own SCRIPT, each tells its own tale of their world.
*"I keep watch of these worlds, ensuring that the SCRIPT is ongoing as foretold."
*...
*"I may share the same purpose as Ink, but I shouldn't let him know I exist."
*"HE.MUSTN'T.KNOW."
*...
*"In any case, this space is now my home... May your AU's and ATs are protected, dear Creators."
*Don't stop creating, keep it up."
(Thanks to @templarskyverse @writersgonnawrite for helping me fleshing out Unknown's story)
His soul is in the Heart Locket. How exactly? He's unsure himself.
His jacket is like a comfort blanket. If he wears it, he'll be calm, fun and playful. Not wearing it or partially on his body (like in the drawing) causes him to be more serious.
The SAVE star in his left eye is a remnant of his quarantined world, now allowed him to access the hidden CodeSphere and gain code related abilities.
One power is COPY, allowing him to copy another Sans's power. But because any Sans au is just basically a copy of a copy, his skill is weaker than the original copy but still cause damage.
He doesn't want to interact with Ink, for some reason. He seems reluctant and worried about consequences.
Unknown doesn't want to meet Error out of safety of his version of Frisk, Chara and Flowey.
He can't SAVE his own world. He could only quarantined it for now.