After a lot of brainstorming, I finally came up with an undertale oc.

Name: Entity (I still can't think of a good name so she'll be called this for now)

Age: ??? (Appears to be biologically 13)

Appearance: she has long black hair, strange blue eyes and wears a striped shirt like Frisk, expect it's pink with blue stripes and has a pencil she balances on her ear (forgot to add)

New update:

Soul colour: No soul but has a fake red soul

Abilities: the pencil she has can be used to "draw" objects for using (eg: fake soul) so she's a bit like Ink. She's not from the undertale multiverse so she doesn't have a soul. As if it was another dimension, her body changed to match the people in the multiverse, explaining the odd eyes with no pupils (sanses + natural blue eyes) and the outfit. She used her pencil to draw a fake soul to match the Frisks in the multiverse.

Relationships: she doesn't want to disturb the people in the multiverse so she doesn't talk to anyone. She just watches from afar out of curiosity. The only few people who notice her existence is Core!Frisk, Error!Sans, basically any universe-jumping characters and multiple people in Outertale

Story: When she first appeared in the multiverse, Core!Frisk noticed immediately but didn't bother her. Some time later while Entity was going through AUs for fun she happened to see Error destroying an AU and Frisk dragging some people through a portal then disappearing. As she was watching from afar, she didn't notice Error saw her until it was too late and Error started chasing her. Out of panic, she grabbed her pencil and tried to draw a portal to safety with the portal Frisk used still in her mind. Once she drew the portal and leaped into it, she found herself in the omega timeline (a bit due to knowing the existence, a bit due to glitching) and met Core!Frisk that way.

She loves going to Outertale as she finds it having a feeling of home (she can't seem to go back to her old multiverse) and was often seen by the people there, although they leave her alone.

Extra: Everyone who first saw her thinks that Entity is some other AU Frisk. In reality, she's not one, but just happened to look like one

I'll add more things to her someday