Replacetale is a switchup AU made by tumblr user TheFunHasBegun and their brothers.
- Alphys is the role of Flowey. She sends "friendliness lightning", which is actually harmful bullets.
- Asriel is the caretaker of the RUINs.
- Sans is a depressed skeleton encountered in the RUINs.
- Mae is a Sentry in Snowdin, Waterfall, and Hotland, as well as a burger vendor in Hotland. She is the final boss in the Genocide Route.
- Red wants to capture a human so she can join the Royal Guard.
- Mettaton wants to see his hero and cousin, Napstablook, in action.
- Napstablook chases the protagonist throughout Hotland. He is the head of the Royal Guard.
- Monster Kid is the Royal Scientist, and creator of Papyrus.
- Undyne is Monster Kid's assistant.
- Scora was the Royal Scientist before MK, and the creator of the CORE.
- Papyrus is the star of the Underground, and the brother of Sans.
- Toriel was a good friend to Asgore before she died. MK managed to bring her back, as a flower.
- Asgore possesses you in a Genocide Run.
- Chara is the Ruler of the Underground.
- Bratty and Catty own a diner in Snowdin, which is Mae's favorite place to eat at.