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Underswap is an AU where all of the characters' roles are swapped, as well as their personalities, though some quirks tend to stick in places, or are modified. Sans is the excitable, confident, extremely optimistic human hunter, while Papyrus is the laid back, protective bro following Chara (who is swapped with Frisk). Other swaps in this AU are Asgore and Toriel, Alphys and Undyne, Napstablook and Mettaton, Grillby and Muffet, Burgerpants and Nice Cream Guy, and Flowey and Temmie.
The plot of Underswap revolves around each character from the Undertale universe being swapped with other characters, such as Sans' and Papyrus’ personalities being swapped (although they keep the same names, so what we consider Sans to be as a character is now how Papyrus is, for example). All of the songs are remixed to swapped characters as well.
- Sans has been swapped with Papyrus.
- Alphys has been swapped with Undyne.
- Toriel has been swapped with Asgore.
- Grillby has been swapped with Muffet.
- Mettaton is swapped with Napstablook.
- Flowey gets swapped with Temmie.
- Frisk gets swapped with Chara.
- Burgerpants swapped with Nice Cream Guy.
- Temmie is swapped with Asriel.
- Aaron has been swapped with Shyren.
- Monster Kid is swapped with Asriel.
- Mad Dummy is swapped with their cousin, Dummy.
- Bratty and Catty swapped with RG 01 and 02.
- The Annoying Dog remains the same, except... Toby is swapped with Morty.
- W. D. Gaster is swapped with River Person.
- Monster Kid is swapped with W. D. Gaster.
- W. D. Gaster is swapped with Gerson.
- Asriel is swapped with Chara.
- Temmie is swapped with Annoying Dog.
- So Sorry is swapped with Glyde.
- Froggit gets swapped with Whimsun.