i think the reason she fears him so much is because there's a difference between inside the painting where he feels human and outside the painting he doesn't. i also think it doesn't help she was immediately interrogated by strong hunters.
i think it will just take some time to get used to jio for them! he's just kind >_<
It's the equivalent of meeting an eldritch god. Just because the entity is friendly towards humans doesn't mean its idea of friendliness is the same as humans. What if the entity think ripping your face apart is a nice reward to those who think themselves ugly because they wouldn't have to look at it in the mirror anymore for example ? An horrific act done out of kindness is a common trope in cosmic horror.
And even if the impossible to decipher abomination showed friendliness it doesn't means it's actually benevolent. Again, in horror it's classic to have an entity befriend humans and help them for malevolent purposes or by having hidden costs for its help.
We know as readers that Jio is a good and kind person but they have no way of knowing that, remember that they instinctively recognize Jio as a threat to their life, they KNOW Jio could kill them whenever he feel like it.
It's like meeting pennywise in his sewer and he is handing you your paper boat back, do you take it or do you run away ? And if he were to truly give it back and not devour you on the spot, would you then invite him in your house and be happy to see him on the street or would you wish that someone took care of that creepy supernatural clown and made sure he cannot hurt someone ?
To add to what the replies are saying, i believe another reason why she feels so afraid is because of the difference in their levels (?) Like shes a C rank hunter, and even the A rank hunter still feel weary of Jio. Its unfortunate but hes like this all powerful being who they still cant put a measure on his powers and strength. I feel like if Jio sat down and told them his whole history like his birth records, where hes from and his school history, i feel like they'd believe him more.
I do not like Hunter Cha Ara... In my opinion she shouldn't be so hostile to Jio, Jio saved her, fed her and even healed her wounds. I know she felt uncomfortable around him but that shouldn't be an excuse to behave like that around him as if he's a heartless monster who will eat people, like what do you mean by "lock him up"? He was able to save her because he is not locked up... My heart aches for grandma Jio who likes nothing but eat and feed those who are around him.