-TORIEL
If you accidentally scare Toriel, she'll jump before laughing.
If you scare her on purpose, she'll do the same thing except she'll scold you while she laughs
"(Y/n)! I didn't see you there!"
-SANS
You don't usually get an opportunity to scare Sans.
But when you do? Oh man, it's amazing.
If you accidentally scare Sans, he'll scream and teleport away.
What's funny is that when he screams, he'll cut himself off by teleporting
"aa-!"
-PAPYRUS
Papyrus doesn't get scared too often, but he also does at the same time.
Like, the only time he gets scared is during Halloween.
So the only time you'd be able to accidentally scare Papyrus is during Halloween.
If you jumpscare him by accident, he'll jump and defend himself in a weird pose before realizing it's only you
Like of like the way Susie poses when Noelle asks her what she's doing in Deltarune Chapter 2
"AH! OH! (Y/N)! YOU STARTLED ME!"
-UNDYNE
Undyne is practically unable to get scared.
The only thing that can scare her is letting everyone down (Which she doesn't need to worry about anymore) and dying.
You can't scare her.
"HEY (Y/N). WHAT'S UP?"
-ALPHYS
Alphys is scared of a lot of things.
She's very easy to scare.
But you try not to scare her too often.
Because there was one time when you scared her while she was carrying ramen noodles.
She jumped and the ramen flew out of the bowl it was in and landed on her head.
She had a bad burn on her head for about two weeks.
"I-I-I'm okay! It just...r-really hurts..."
-METTATON
Mettaton, being a big ass robot.
Is pretty hard to scare.
Almost impossible.
If you want to scare him though, wear a horrible outfit and show it to him.
He'll have a heart attack.
"AH! DARLING! WHAT ON EARTH ARE YOU WEARING?!"
-ASGORE
Asgore is pretty easy to scare.
If you scare him, he'll jump and hide his face in his hands.
Before laughing when he realizes it's you.
"Oh! (Y/n)! You spooked me!"
-MUFFET
Muffet, being a spider, is also pretty hard to scare.
The only thing that scares her is loud noises.
If you drop like a bowl or something and it makes a loud boom sound, she'll jump and scream.
"AHH! OH! Dearie! It's just you! Sorry, you frightened me."
-GRILLBY
Grillby doesn't get scared easily.
But he is afraid of water.
Don't try to scare him though, he'll get very mad at you if you hold a glass of water near him.
"...keep that away from me."
-GASTER
Gaster, as pathetic as it sounds, is afraid of the dark.
It's because the dark reminds him of the void he was trapped in for so long.
So if you want to scare him, turn off the lights.
He'll get very mad at you if you do that though.
"❄︎♒︎♋︎⧫︎ ⬥︎♋︎⬧︎ ☠︎⚐︎❄︎ ☞︎🕆︎☠︎☠︎✡︎!"
(Translation: That was NOT FUNNY)
[Hope you enjoyed. Stay tuned for more and have a good day]