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 but comes across as arrogant, assertive, aggressive, a smoker, perverted and flirtatious