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IntroShiftTale is another swapping AU. Need I say more?
Flowey stays exactly the same as original, except he is now a she and she uses 'friendliness bolts'.
Papyrus is the caretaker of the Ruins. He is still fun, still loves making spaghetti (even though it's bad) and he still loves puzzles. However, despite the fact that he has (mostly) his original personality, there is still a hint of Toriel's personality as well.
Sans is the same, pun-loving goof everyone loves. He mainly hangs around the Ruins with his brother (hence, he takes the place of Napstablook). However, he is extra lazy and can't dodge your attack (in the Genocide Route).
While Napstablook is still depressed, this has subsided in favor of Sans' personality. (Although is depression is still there.)
Mettacrit takes the role of Papyrus, wanting to be a TV star with a robot body.
Undyne may take the place of Monster Kid, but she still has the Undyne personality. She doesn't have the Undyne STRENGTH, though... She also often calls the protagonist 'punk'.
Captain of the Royal Guard, Asriel will hunt you down. He doesn't wish to hurt humans, but he has too, to keep his position as Royal Guard.
Toriel keeps her original personality, but is now to Royal Scientist.
Asgore fell into the lava of Hotland, being saved by an unknown entity. He was badly burned, but he had robot parts installed by Toriel. He is the star of the Underground, and tries to hurt the human. (It is later revealed that Asgore had forced the Mettaton personality because 'it's what the fans want'.)
Monster Kid (now called Monster King) is the ruler of the Underground. He has mainly Asgore's personality (but a little of his own), and doesn't wish to harm the human.
Chara was the Royal Scientist before Toriel. She fell in the CORE and was shattered across time and space.
Gaster takes the place of Chara. Gaster is a calm yet psychotic genocidal megalomaniac who hates both humanity and monsterkind.
Alphys takes the place of Asriel. She possesses Flowey (as revealed in the True Pacifist Route). You 'fight' her at the end of that route, where she becomes the Goddess of Hyperdeath.