In Undertale, Sans is a short and big boned skeleton, with a perpetual grin on his face. He wears an unzipped hoodie, a white t-shirt or sweater, black shorts with white stripes and a pair of sneakers or slippers. He has white pupils, which tend to disappear when he is serious. When fighting, his left eye flashes cyan and yellow and his right pupil disappears.

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.

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, Chara, also known as the First Human or the Fallen Human, is the first human to ever fall past the Barrier on Mount Ebott into the Underground. This person is who the player names at the start of the game, and not the controllable character who is played throughout the entirety of Undertale. Chara is seen to be wearing a lime and cream colored striped shirt, brown pants and shoes. Compared to Frisk, Chara is of a lighter skin, hair color and has red eyes.

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