I will assume you meant Undertale!Frisk without the specification, but feel free to correct me if I'm wrong ^^/

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Undertale!Frisk - Frisk is a bit of a chaotic individual, so instead of being a sappy romantic when they see you in their sweatshirts or thinking it's really cute... well, now they're going to make a habit of wearing your clothes as well. If you keep going along with it, well, eventually it'll just come to a point where neither of you will know whose clothes are whose anymore (ESPECIALLY if you guys are the same size, it would probably be easier to tell if one of you is larger than the other). Needless to say, any time you guys go out clothes shopping from here on out? You guys are just getting a bunch of things you both like and can both fit into. Also, clothes gifts from now on would be pretty awkward. It's like when someone buys a game for their friend so they can play it together but the friend who bought it actually wanted it more than the one they gave it to... but with clothes.

Kris - Of the humans I write here, Kris is without a doubt the quietest of the bunch. They’d also the least expressive and doesn't really express enjoyment or lack thereof, even when in peaks of emotion. But even still, if you decided to take it upon yourself one day to wear one of their sweatshirts, oh. Well, how to tell you they're super attracted to that? It's incredibly cute. Like the cutest thing they've ever seen. They probably won't say anything directly, but they’ll probably give you a hug the moment they see you like this, no matter if you're in the middle of doing something or not. Bonus points if you're taking a nap or are curled up on a couch watching TV or something of the like -- now it's a cuddling session. Oh, and if you were asleep, Kris doesn't really care if they wake you up or not. This is more important.

Undertale!Chara - Nngh... I hate it but it looks cute. -- the summary of Chara's current state of emotions. They aren't the biggest fan of you wearing their clothing, to be honest, but they have to admit you do look cute regardless. But they don't know why you're wearing it. Why do you need to? You have your own clothes that are perfectly fine, yes? Chara's going to probably ask you to wear your own clothes immediately, despite being red in their cheeks. If there is some specific reason that your clothes are not available -- in the wash, need replacing, etc -- they'll tolerate it for a little while, but you better believe the second you have some clothes available that they'll tell you to wear them instead. Though, you have to wonder if it's because you in their clothes was distracting them more than they didn't like it.

Underswap!Chara - Practically immediately gives you their wardrobe. But in return, takes your wardrobe. Reacts pretty similarly chaotic to UT!Frisk in this regard, but Swapra is going to be way more flirty about it. Also you better expect some more frequent cuddling sessions as a result of this. Why? Well, there's really no particular connection between the clothes-swapping and the cuddling other than Swapra just really likes being overly affectionate and found this to be the absolute best excuse to cuddle more often. Sharing clothes? Next step, sharing body heat. They also might be a little more physically clingy and huggy. If you wanna get any work done, hopefully you can do it outside of the house, or Swapra is definitely not going to let it happen.

Chara : The Fallen Angel

Undertale!Gaster - He doesn’t really mind. Gaster’s pretty laid-back with most things and he’s willing to be patient with someone who has difficulty understanding emotions. They are a difficult concept after all, so no need to treat it with such seriousness. He’ll be pretty chill about the whole thing and really won’t worry one way or another. Though, he’s not one for serious conversation, so don’t expect him to sit down and have a wholly genuine, joke-free conversation about this pretty much ever.

Undertale!Chara - Possibly shocking, possibly not -- Chara isn’t a fan of this. While not exactly someone empathtic themself, they do kind of expect other people do be so, or at least have some sort of ability to recognize Chara’s desires without them having to really say a whole lot. Chara’s a very emotional person with very little impulse control, so frankly, it might be a bit between someone who can’t communicate their emotions well and someone who can’t understand them very well.

Undertale!Asriel - Asriel understands you completely. He couldn’t feel any emotions as Flowey, much less even so much as sympathize with anyone else. He’ll be the most proactive of this collection to try and help you do everything you’d like to do. With his own desire to better himself as a person, he would love to do a lot of good deeds and other various positive impacts on others’ lives. The potential biggest issue is that Asriel himself is a very emotional person, but he’s also used to being around people who don’t necessarily recognize or reciprocate that.