troublesome description

I'm trying to figure out a good way to describe some characters.

They trigger a reaction sort of like the "uncanny valley" effect, except instead of weird/creepy it's more scray/terrifying.

Sort of like something about them makes your lizard brain want to scurry away and hide, except it's afraid to move because it might attract their attention. :-)

There's nothing you can point to that causes the reaction. It's not any one detail or details. It's an "overall" thing.

They look "normal" or maybe better than normal. But the first reaction is as described above.

Basically, I'm trying for "idealized humans" except for the "scares the hell out of you" initial reaction.

It's something you can get used to, but that first "jolt" ...

And yes, they frequently start out by saying something like "fear not". :-)