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Frisk's role is filled by Alphys. Still shy and awkward, she is more talkative than Frisk is in UnderTale, but still isn't quite willing to open up to someone. She secretly wants to kiss Undyne.
Flowey's role is filled by a black flower named Buster, representing Stable Gaster. A tactical and statistical murderer, when angered, he forgets science and attacks. His relationship with Sans is... "complicated".
Toriel's role is filled by Sans.￼ A lazy and apathetic skeleton in a baseball cap, he takes care of the Ruins because it's the one place that reminds him of Papyrus. His origins are intentionally clouded in mystery.
Ruins Dummy's role is filled by Toriel. A kind and positive mother figure, her main problem is that she's too overprotective. She is not in love with Sans, but they are close friends.
Napstablook's role is filled by Frisk. A shy, yet powerful human, she lives in the Ruins with Toriel. She wears Sans's socks like boss monster ears and hides under a cloak to make others believe that she is a monster, as if anyone were to find out that she is human, it would knowingly cause mass civil war.
Sans's role is filled by Muffet. A descendant of the near extinct "spider" monster species, Muffy is rather apathetic, but unlike Sans, she knows when to strike. She loves to bake pasties, but never lets surface dwellers try some. She lives with her adopted brother, Grillbz, in a large house in Snowdin.
Papyrus's role is filled by Grillby. A flame monster with no clear motives, Grillbz is a creature that keeps calm under pressure and is determined to catch a surface dweller like a Pokemon. He is also not a bartender in this AU.
Grillby's role is filled by Chara. A descendant of the human family raised by the ancient rulers of the underground, Chara runs an informal restaurant in Snowdin. He is not evil in this AU, rather he is a being always ready to lend a helping hand.
River Person's role is filled by Mad Dummy. Not much to say in terms of personality or motives.
Monster Kid's role is filled by Undyne. A very powerful and dangerous being, she has been training to become part of the Royal Guard ever since Gerson retired. She secretly wants to kiss Alphys.
Undyne's role is filled by Mettaton. The most glamorous fighter ever to live, he treats every fight like a dramatic scene in a movie. He became the Royal Guard Leader instead of it falling onto Undyne because Undyne was too young at the time of Gerson's retirement. He wields a chainsaw.
Gaster's role is filled by Ruins Dummy. Not much to say other than that he is much more talkative then Gaster and was a biologist in his life.
Mad Dummy's role is filled by Asgore. Once king of the underground, he retired out of pity for all monsters after having killed the Patience SOUL. He truly misses his family, but due to his guilt, he is unable to think rationally enough to come to terms with Toriel. He lives in Waterfall with Undyne.
Alphys's role is filled by Napstablook. A specialist in ghost science, Blooky is the owner of the lab in western Hotland. He is a shy spectre, unable to face the consequences of creating the Amalgamates directly. His favorite food is ghost sandwiches.
Mettaton's role is filled by Monster Kid. An extremely powerful and optimistic being, MK is a creature that doesn't keep calm under pressure. Despite this, he is a mega star and has a bigger fan base than Jacksepticeye. He was best friends with Frisk before he became the star he is today.
Muffet's role is filled by Asriel. A cheerful boss monster, he runs a small shop in Hotland where he sells sugar cookies to raise some money to get his family back together. He is always happy unless he has to meet up with Asgore or a surface dweller. He was taught in the art of baking by Toriel.
Asgore's role is filled by Papyrus. The somewhat optimistic king of monsters, he is a cheerful skeleton and Sans's brother. Despite his positive nature, he takes his role as king of monsters very seriously. He fights using bones and Gaster Blasters.
Asriel's role is filled by Unstable Gaster. The biological father of Papyrus and previous magic specialist, Gaster was slain many years ago by the surface dwellers when bringing Angel's body on to black flowers. Like Buster, he thinks similar to Fi from Skyward Sword in that he says everything based on statistics.
Chara's role is filled by River Person, named Angel. Not much to say other than, pure evil.
FlowersAndInternet06 from Discord fills a new role, replacing when you meet Blooky in his house in Waterfall, as Frisk lives in the Ruins with Toriel. An optimistic being, this flower monster takes great joy in making others feel good about themselves. Her SOUL is a mix of human and monster SOUL structure, and the unbalance has been known to cause her SOUL to act on its own.