Fell Sans is a skeleton who is pushed around by his younger (albeit significantly larger and more durable) brother Papyrus. One difference about Underfell Sans is that his eye glows red. Fell Sans, much like every other monster in Underfell, is a darker and edgier version of the original Sans. Sans can be harsh at times. He is not into puns that much, but he likes knock knock jokes. He often gets tired, but not flustered. He has an allergy to cats. He strives to be cool and collected.