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 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 Undertale, Grillby is a quiet fire monster who owns and runs the bar Grillby’s in Snowdin, where Sans is a regular.

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.

Underfell Sans is special kind of special, a murder-y kind. He enjoys long walks on the beach and inflicting pain on the human. Being pushed around by his brother Papyrus, this is a given.

In Undertale, Undyne is a fish-like monster that leads the Royal Guard. Clad in her full armor, she pursues the protagonist through the entirety of Waterfall, frequently evaded or inadvertently thwarted by the Monster Kid. She has blue scales and a long bright red ponytail. She has red and blue fins on the side of her head, and a pair of sharp yellow protruding teeth. She wears bright red eye shadow and has an eyepatch on her left eye. Her eyes have vertical black pupils and a yellow sclera.