As someone who doesn’t read the novel

As his assimilation/synchronization with the personalities increases, the more apparent they integrate with him.

But the others don’t actually see his real or other appearance right? They only see the dark ‘monster’ version whenever Jio is out of the painting?

They can see it. I believe it depends on the level of power. but sungwoon sees and also notices when Gio turns into Giovanni

Correction: Gio decreases his aura and presents himself in a human form to avoid drawing attention, so his aura appears threatening, but his appearance is normal. In the novel, it's often mentioned that his face is hidden by the cloak and sometimes when he turns blond nobody notices bc of it