To contextualize this: I tend to only properly exist during night time (especially midnight) and roam around with all lights out... I also walk without making a sound due to being used to living in apartments, both of those bad habits lead up to me scaring the people I live with rather frequently...
Enough babbling, how would the characters react to them going to grab a midnight snack or heeding nature's call just to get jumpscared by looking behind them (or to a place they looked before) and boom! the Demon Lord is suddenly there!
Damn, with this trait + getting transported to become a anime character, you really could kill someone┬┴┬┴┤(・_├┬┴┬┴
On second thought, maybe, it wouldn't be safe to hug a fox's thigh after all...
The character who would react the hardest haven't appeared yet, but I think their reaction would be pretty amusing.
Yuven: If this takes place at the beginning of the story, this a very straightforward way to kill off the hero and reign supreme as the monarch or all realms heh
Usually, the Hero's reaction to surprises is: Startle-> Fearful->Slaughter If anything makes them afraid, be sure that such s thing won't be alive to do so by the next second. (Don't you remember about what happened to that acorn)
However, they will be sensitive to the Demon Lord's presence after spending some time with the Demon Lord, so they likely won't be startled. (I'm not supposed to reveal this yet, but the Demon Lord's wings drag against the ground+ Yuven uses it to figure the Demon Lord out.)
Attendant A: Attendant A falters where they are standing, but otherwise, there isn't much reaction. The Demon Lord might pickup on the scent of heavy fear on their person, however, when questioned, they show only respect and impassivity mixed with apologies.
Attendant B: Attendant B does not scare out of surprises, they are the most boring out of all the cast if you're looking for a surprised reaction. They blink, and smile at their lord, "My Lord is up so early, would you like for this servant to draw the bath?"
The Sibling: The sibling startles easily, pupils shrinking heavily in size. If this is a habit of the Lord's, they show deep resentment towards it. "You're always so silent, elder." They eye the dragging wings with eyes devoid of emotion, "Even when burdened by noise."