Hmm... Okay, I got one, I think.
This is mostly for UF!Sans, but could also cover his bro and their Undyne. How would they deal with an s/o who is protective of them? As in, willing to pick fights and get mean to keep them safe.
The Fell is a harsh place, and, yeah, most folks can get by just keeping their heads down, but the s/o has other plans. Whether they're stepping up to the plate or just stooping to the Fell's level is up for debate; maybe it's posturing, maybe it's genuine, there's no way to know right off the bat. But, either way, the s/o wants to be the sort of figure their partner can trust to make them feel secure, whether that means cuddling up under a weighted blanket or biting a hole in someone who thought they could pick on a couple of lovebirds.
They can be soft in private, and want to prove they can be equally sharp when needed, so that their partner doesn't always keep their guard up. Strength in numbers, and all that.
Red is a little easier for me to imagine this way, because he's the shortest, whereas Edge and Atra are very imposing and would set a pretty high bar for intimidation. I might be wrong, though. How do you think something like that would pan out?
I don’t really understand the name “Atra” for uf!undyne. It seems like a really good one!! But I don’t get it, so I’ll be using a different name for her!!
That being said, on with answering your request!!
Red can appreciate the protective streak. Like a lot. He thinks it’s pretty hot that, even though he can technically take care of himself, you still insist on being protective. If you try to be the only protective one, he might feel a little... stunted? Like you don’t appreciate that he can take care of you, too. But if it’s a give and take where you to will go in guns blazing for each other? Well, that’s just a match made in hell, isn’t it?
Edge is a little more... hesitant about your protectiveness. He likes to provide, to be seen as competent and capable. And you baring your teeth on his behalf kind of... makes him feel a little worried he misunderstood what you wanted in your relationship with him. He actually ends up sitting you down during one of your private moments to bring it up before it grows into something he resents because he loves you and doesn’t want that to change. The conversation ends with him feeling better and you with a bit of a better understanding on how to have his back without making him feel like he’s not providing you protection. He very much enjoys seeing you get protective after that, knowing you’ll step back if he asks you to.
Fang thinks it’s fucking adorable that you stick your neck out like that for her. Especially considering her position in the Royal Guard and being Asgore’s right hand. Seeing you with this fire in your eyes, passionate about keeping her safe even though she doesn’t need your protection? FUCK YEAH BABY THAT’S WHAT SHE’S TALKIN ABOUT!!!!!! Fang’s pretty good at dragging you away before shit gets too bad, she doesn’t want her time on the surface to be tarnished by you getting in trouble after all. She does make sure you know how much she appreciates your fiery passion later, though.