Achilles’ Heel anon here! I love your answers! And indeed that question remains, for the same characters, how would they react if something DID happen to the weak spot? ((Bonus: what if they were the ones that caused it? Accidentally, of course.))

iimagine-undertale:

So as to avoid getting too angsty, we’re going with the interpretation that getting hit in one’s Achilles’ Heel does not mean instant death (after all, Achilles himself was hit with a poisoned arrow, so…) Also, I like you, you’re mean. Anyhoo!

Toriel would be absolutely horrified- at first. And then furious. Whoever was responsible would pay, and they’d also apologize- unless it was a bigger battle than just a regular encounter. But her biggest priority is to heal you, and get you to safety- invulnerability or not, she is not risking your safety. But if she caused it…? Oh, she’d cry and apologize so much. Of course she’d do whatever she can to fix it and make up for it but how could anyone possibly forgive her for this? She would understand if you wanted to leave her after this- you had told her, and she had ended up the biggest threat…. how could she have done this?

One may not realize it, but Papyrus is an excellent fighter. The thing is, he doesn’t want to fight- he’ll capture enemies, sure, but. He’s not about the violent life, even if he does want to join the royal guard. But if someone hurt you? Well, they’ll see how good he is. After all, Sans isn’t the only one with blasters. And he doesn’t have to help someone get over his attacks. Now, I truly doubt Papyrus could be the cause- his control is simply too good- he can stop an attack in it’s place if he sees that his opponent is too hurt to continue fighting, and this likely comes from growing up with Sans. But if by some horrific miracle he did hurt you? He would never forgive himself and his confidence would likely be shattered. How could he be the Great Papyrus if he managed to hurt someone he loves so much?

Sans is devastated, of course- but he knows the risks of fighting. He can’t avenge you personally, but he can take care of you while you recover, and keep your mind off things. But the thing is? Sans is a popular guy. He’s got friends and acquaintances everywhere. So sure, the one who hurt you might not have a bad time ™ from Sans himself… but don’t think they’re safe. Sans could get the whole Underground after them if he wanted. But if Sans were responsible, you… honestly wouldn’t have to worry. The guy has great control if only because of how his attacks are, and also only 1 attack point. If anything, it might tickle. But dang if he didn’t give himself a heart attack. (He didn’t even think it possible!) 

Undyne would break the attacker. Not just bones, the whole attacker. She’d make them rue the day they even looked at you, spitting fire and vulgarities the whole time- but then, she’d just feel guilty. She swore she’d protect you, and that one vulnerability. She trained so hard, and for what…? To- to fail you? And oh, the guilt would be even worse if she were the one responsible. For a while, Undyne would even lose her will to fight. But as the captain of the Royal Guard, it’s not like she can just give up… Forgive her if she avoids you when she trains from now on. She- she just can’t risk it again. She loves you far too much. 

Alphys is horrified, of course- horrified and distraught and confused. You’re invulnerable, your only weak point is well protected, she made sure of it, you’re- you’re YOU. How could this have happened? Undyne and Mettaton are more than willing to help avenge you (”I’m not dead,” you tell them. They don’t care. The one who did this is gonna pay.) but ultimately, Alphys isn’t a fighter. She’s going to take care of you though, don’t you worry. And while you recover, she can show you some new shows. Wouldn’t that be nice? If she were the one responsible however, she’d- she’d be so crushed. Any progress she’s made would revert back to zero so hard, and she’d avoid you for a while. Needless to say, she wouldn’t handle it well. 

We all know what Asgore’s like when he’s mad- and it’s only the fact that you survived that spares your attacker (who may or may not need a few therapy sessions now). He’s super reluctant to leave you alone at any point now, even after you recover- it’s not that he doesn’t trust you, but he’s- he’s lost so much already. He can’t lose you too. And if he were responsible, he’d hate himself. … Even more, I mean. He would not use his fire magic, he would not touch his trident- I doubt he’d even touch you, for a while, even to hold your hand. He’d beg for your forgiveness- even though he feels he doesn’t deserve it.

Gaster would be fairly pissed, to put it lightly. He may not be a fighter but he IS a scientist and everyone knows how to get… unethical about it. Needless to say, your attacker would not bother you again. Why, it’s almost as if they’ve vanished completely… only to come back some time later, possibly a bit traumatized. No, no, don’t you worry about a thing, you will not be bothered again. … In truth, I can’t really think of a way for Gaster to cause it himself, save for perhaps indulging in his curiosity eventually- though the moment he noticed he actually hurt you the experiment would stop faster than you can say “ow”. He’d fret so, so much, beg your forgiveness- his curiosity was not worth putting you in danger, and he will NOT forget this lesson any time soon.

Grillby would be furious. Do you know whats scarier than a fire guy staring at you silently? A fire guy staring at you silently but he totally wants to kill you. Thankfully, he has better things to worry about- like getting you to safety. As soon as that’s taken care of, he’d be damn sure to find the one who did it and ensure this never. Happens. Again. If it had been him, however, Grillby would be horrified- he always takes such care to avoid this, after all. He wouldn’t leave you unless you wanted him to, but he’d handle you like paper, or glass- as if dropping you could break you into pieces. I doubt he’d ever forgive himself. 

Muffet is… not one anyone would want to anger. And hurting you? Her darling? Oh yes, that would anger her quite a bit. Needless to say, she’d have something new for her next batch. And then she’d dote on you- have you been resting? Do you need another cup of Spider Cider? Perhaps a donut? Monster food heals you, be sure to eat up. Because of her own people’s anatomy she may not react as extremely for long, but she’s very protective of you afterwords and will not risk you so much as catching a cold for a while. If she herself caused it she would be furious with herself- how could she have been so careless?! Of course she’d ensure your recovery, but rest assured it will not happen again. She would never forgive herself. 

Mettaton would be so very angry. He’s the star of the Underworld, and you, his nebula- and someone had the nerve to hurt you? Congratulations, they have invented a new kind of stupid. Now, he’s not merciless to kill someone because of that. But you can bet he’s hired all kinds of mercenaries to hurt them for him. He can’t take care of it himself, he needs to get you to a healer as soon as possible! And then he needs to dote on you and recline seductively for your viewing pleasure, so you don’t have to look at some dreary, bland hospital wall while you recover.  But if he had been the one to do it- he doesn’t understand. You’re invulnerable, aren’t you? How- how could this happen? How could he have made such a mistake? He’ll do all he can to make up for it- he’ll spend whatever gold he must, do whatever you ask, anything- just please, please forgive him….