How do you think the fell bros + fell grillby and the sf bros + sf grillby react to finding out their beautiful crush is secretly a glamoured Huldra woman (Norwegian mythical creature with a cows tail and hollow back). They find out when she accidentally grabs something red hot but is totally fine after dropping it. Maybe she's so startled by slipping up that her glamour (that makes her look human) falls to reveal her ethereal appearance! :)
(Quick google search revealed that Huldras are probably connected to Frau Holle, and since I loved the fairy tale about her I’m super glad about this information
UF Sans: He is…scared. At first. He is fine with surprises, but his initial reaction is fear, so give him a few moments to get his bearings again before you explain what you are. He is….pretty ok with it. Like, he is a walking, mustard drinking skeleton, if you where ok with that, he is ok with you having a tail and not beeing hurt by hot things. He is a bit disappointed that you haven’t told him before, but he gets that you probably had a reason.
UF Papyrus: He let’s out a squeek. And actual squeek. He is pretty easily startled, and you grabbing something extremly hot and then suddenly changing into your true form is pretty damn startling. Explain him what you are in the fastest way possible, he doesn’t have the patience to listen to long explenations and migth panick if you don’t reassure him that it’s still you. Otherwise hes fine with you beeing who you are, he will be a bit grumpy for the next few days because you didn’t tell him earlier (and because he is embaressed by his reaction).
UF Grillby: “What the fuck!” His flames flare wildly, he is already in a defensive stance. But he get’s his cool back pretty quickly and gives you time to explaine. Afterwards he will chuckles sligthly, you really manged to surprise him, that’s not an easy thing to do. He grows pretty fond of your tail.
SF Sans: He is all over you in an instance, he is so floored by yoir appearance. And especially by your skill to touch hot things without sustaining any damage. He wants you to become his battle buddy, you two would be unstoppable in his opinion.
SF Papyrus: He is so fascinated by you true form, the nerd in him is screaming. Please tell him everything about yourself. But he wont pressure you, trying to keep his cool, but his eyes are shimmering in excitement.
SF Grillby: He proptly stumbles over his feet. However one manges to do so while standing still. He will first and foremost make sure you are actually unhurt, he knows exactly what nasty wound extrem heat can cause. After that he studies you with a warm gazy. He is very fond of different creatures, he has a few fiery pets that he acquired because of that reason. That doesnt mean he will treat you differently, but at least he stopps worrying that he could hirt you if his control over his flames slips