Storyswap - Since this won't be a full fledged AU, I'm not changing the name
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Chara -> Papyrus
Asriel -> Sans
Sans -> Toriel
Papyrus -> Asgore
Alphys -> Mettaton
Undyne -> Napstablook
Toriel -> Alphys
Asgore -> Undyne
Mettaton -> Asriel
Napstablook -> Chara
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Sans:
Sans is the "caretaker" of the Ruins. Caretaker in quotes because he does the exact opposite, the Ruins are a horrible mess. He seems to know more than he lets on, something other than a casual, lazy joker. He came to the Ruins to kill the humans that fell down before Papyrus, but by the time Frisk falls down the burden's been hitting hard and he can't bring himself to do it. When Frisk tries to leave, he knows there's only one thing left to do. Still keeps in contact with Papyrus through phone, but he doesn't ever get to see him.

Papyrus:
King of the underground, except hardly. He's blissfully ignorant of what's truly going on with the royal guard and Sans. Killing the humans was not his plan at all, in fact the ones he has met he always welcomed into his home with open arms. ...They always mysteriously disappeared. In fact, when you ask about the barrier, Papyrus leads you to it, though he knows the only way for you to truly leave is if you're strong enough. So, he challenges you to a battle. Guess how that ends.

Chara:
Chara came from a village that basically shunned them as a "demon" for their Red SOUL. As a result, they tried to end their life through Mount Ebott. They ended up landing in a hole in the middle of Snowdin Forest, where a younger Asriel had luckily passed through one day. Asriel brought Chara to his parents and Chara was quickly adopted into the family.
Chara and Asriel live on their own in Snowdin, as it ensures Chara will stay safe. Chara's set up traps and such for if another human ever does fall, since they still have their hatred for humanity (however, it isn't nearly as strong because of the influence of from the Dreemurrs). The traps were intended to be deadly, but Toriel and Asgore wouldn't allow it. Chara still tries to stop you at the end of Snowdin, but will refuse to kill you only because of the Dreemurrs. They use flower styled attacks, a knife, and flames. They'll also taunt you throughout the fight.
Depending on your actions, the fight will end one way or another.
- Flirting will fluster Chara too much to continue, but the attacks will get harder because they're angry that you aren't taking it seriously. They'll reluctantly accept date as you get more throughout Waterfall, as they needed some convincing from Asgore.
- Pleading will cause the fight to end quicker, because they'll see you don't actually want to fight. The hangout will be out of pity for you.
- Insulting will get them to end the fight out beacuse they are demoralized and reminded of their past at home. You won't be able to get a hangout, and the rest of the run the Dreemurrs will merely tolerate you rather than respect.
- Other ACT option ideas here

Asriel: (Oh boy here we go)
Asriel is a bit more complicated. At a younger age after Chara had fallen, he started getting more and more interested in magic and SOULs. When his mom wasn't looking, Asriel would sneak into her lab and get as many textboxes and stuff as he could. Of course, he didn't understand them at first, but through practice and studying he managed to grow stronger and stronger. However, one day his spells backfired without him knowing. It amplified his power immensly (it gave him the idea for the Hyperdeath OC) and he started to develop his own types of magic (chaos saber, hyper goner), but at night he would dream of other timelines. Specifically of himself in other roles, including canon. Having all this knowledge started to break him, and the more "Flowey" personality would start developing. He could still be kind and caring, but he quickly lost his temper and was often sassy and/or sarcastic.
He's still the Judgement Fight, however there are a few twists. Asriel is terrified of you, because he's aware that if you keep going, he might not ever exist again. It's his final battle. He doesn't want to die, and certainly not for good. So, he manages to find the six SOULs and absorbs them during the battle, going into a slightly weaker Hyperdeath form. You cannot RESET from this point on, but you can still revive.
He still has his normal attacks from UT, just in a much harder variation. Though, he also has seemingly out of place magical attacks that he learned from the various textbooks and blah. These might be water magic, orange/blue attacks, or cloning. Still unsure.
You can still kill him, but not until the very end when he breaks down. His defense will drop down to 0, and he'll dust without another word.