In Undertale, Alphys is the scientist who helps Frisk find her way through the Underground. She’s a dinosaur-like monster who loves anime (and Undyne) and she’s very shy and doesn’t have much confidence in herself. She usually wears glasses and a lab coat, but if you wish to cosplay her dating outfit, she wears a black dress with white polka-dots.

In the Undertale AU, Error Sans is the destroyer of AUs, in opposition to Ink Sans. He is insane, bent on destroying everything he considers a glitch in the multiverse, using blue string to tie up and shatter souls.

In a certain Undertale AU, Underswap, where two character’s identities are swapped. Underswap Sans has the personality of Undertale’s Papyrus. Sans, known as “Blueberry”, is the younger brother of Papyrus and a major character in Underswap. He is agreed, by many, to be adorably cute.

In the Undertale AU known as Underswap, Napstablook is the one who end up as a robot called Napstabot, instead of Mettaton.

In Undertale, Frisk is the playable character who attempts to escape the Underground after falling down and return to the Surface. Frisk is the last of the eight humans to fall into the Underground after traveling to Mount Ebott.
Frisk is depicted as a young human child of ambiguous age, gender, and ethnicity who wears a striped light magenta-and-blue shirt, blue shorts, and plain brown shoes, and has medium length straight brown hair, short bangs, and a blank expression.

In Undertale, Burgerpants is an employee at the MTT-Brand Burger Emporium, a fast-food restaurant located inside MTT Resort. He is a 19-year-old orange cat-like monster who wears a small hat with the letter M on it, as a part of his job working for the MTT-Brand Burger Emporium. Burgerpants is a highly stressed-out service employee, who hates his job and his employer.