I haven't finish the novel but from what I've listened, those hunters extremely anxious to Gio because his portrait survived the great catastrophe and S class appraisal hunter wasn't able his true identity/power. They fear him because he is unknown identity. Some probably viewed him as a disaster and on further chapters he was viewed as god because of his holiness ( related to Giovanni). Then there his personality too , he acts a bit a god-like. but he is a just bit introvert and his not used to communicate with people as he lives in the Portrait for to long. It made the people always misunderstood him.
Sorry for messy reply but I binged this story today, I haven't had a sleep but this is good over all
Massive Novel Spoiler Ahead!
I haven't finished the novel yet, but this is what I understood for now. Sorry for the long ramble ahead.
In the novel, his aura was already kinda explained at this point in the story, but for some reason, they left that part out of the manhwa, or maybe they're gonna bring it up later.
Anyways, according to the novel, every person has a set "essence" (their soul/aura level) that is basically one of the key factors to determining their hunter rank (S, A, B, C, etc.). The level of your essence is something you're born with, so it can't be changed under normal circumstances.
Now, Jio's essence is so vast and endless that the human mind can't comprehend it, which plays a giant factor in the fear and uneasiness people feel around him. There's just so much of it that the human system that gives you your rank can't categorize it.
And this is all due to a massive coincidence, because Jio did actually awaken like a normal guy when the school burned down, and as I understand it, his essence would, under normal circumstances, not have been anything inhuman, but when he awakened a "miracle or miracles" (quoting the novel here) happened, and his soul/essence fused with the Root/Origin.
Jio became part of the Root/Origin, which is the driving force behind the system, dungeons, monsters, awakening, and pretty much everything else. The Root/Origin is something way above human comprehension and the laws of the universe, so Jio, as part of this thing, pretty much feels just as eerie and incomprehensible as the Root/Origin itself.
To people who read the novel or if there is, couldnyou spoil me as to why people outside the painting or normal people sees Jio as terrifying? Like the scary aura (besides the fact that they think of him as ghost). That kinda confuses me always when it happened