Blendertale is an Undertale AU written by Fanfiction.net/Tumblr user KrispyBaconator. It is a role shift AU in the same vein as AUs such as Storyshift. The story is currently being written on Fanfiction.
About the AU
The premise of Blendertale is that one change in the backstory of Undertale catalyzed a chain reaction that caused most of the characters to take on different roles. What that change was has not yet been revealed. The story is written from the point of view of Frisk, who is afflicted with amnesia and can't remember anything from before their fall.
In this AU, the fall down Mt. Ebbott gives Frisk/the Human amnesia, not remembering even their own name. Flower later christens them as "Tori", which is the name they use for the majority of their journey through the underground. In this AU, they speak and have their own personality, a la Inverted Fate.
This AU follows the "NarraChara" theory, leading to the character in Chara's original role to become the voice in "Tori"'s head, giving them advice and offering their own opinion on things. The story refers to them simply as Voice.
Flower the Flower
Flower is... well, a flower. He gives off a warm, welcoming appearance in the beginning, even naming the new human once he learns they have amnesia. However, he drops the act and takes on his true personality once he has them in his clutches, only being stopped by the caretaker of the Ruins.
Alphys is the caretaker of the Ruins. Her weapon of choice is a science-fiction-esque laser blaster, which she uses against Flower and later "Tori". She can often be found fixing up puzzles in the Ruins, or watching anime in her house with a bowl of Ramen in hand. She constantly breaks into fits of anxiety, especially during the fight against her at the end of the Ruins.
Chara is another human that hangs out in the Ruins, helping Alphys with puzzle maintenance. They first meet "Tori" after accidentally trying to kill them during a nightmare, and making it up by escorting them safely to Alphys's house.
Little is known about Queen Undyne, only that she has it out for the last human soul, and that Alphys seems afraid of her.
Mettaton is still the main idol and star of the underground, but now he works out of Snowdin alongside his cousin. He has no desire to harm humans, only wishing to entertain his people to the best of his ability. However, he still has a massive ego, as well as his signature slick hair and pink boots. He also runs a fashion line called Neo Metal Glamour.
Napstablook is Mettaton's cousin who recently joined the Royal Guard for reasons unknown. Mettaton is constantly trying to cheer him up, and a human seems to be the perfect way to do that.