Classic Sans

Sans is shown to be very laid-back and flippant, sleeping on the job as often as he takes breaks. He does just enough work to avoid being fired by Undyne. He enjoys making bad puns, usually relating to skeletons, to the annoyance of his brother. He is kind and reassuring, but becomes eerily serious at abrupt moments (particularly when angry). He deeply cares for and admires his brother, Papyrus, and reads him bedtime stories. He also leaves a dirty sock in their living room for Papyrus to fuss over. Sans worries about others hurting Papyrus or taking advantage of him.

He is also very observant, and his lines and actions change the most through the path and the way of playing the game. He knows the number of times he had defeated the player (in Genocide Route). He tends to say he hates to make promises, this is likely a result of having to harm a human. His laziness is noted again during the final fight in the Genocide Route - he is unsure if being somewhat aware of the timeline resetting has broken his spirit, knowing that everything he does will mean nothing... or if he's just using it as an excuse to slack off. He also seems to be quite patient, as shown by his willingness to use his "Special Attack" (which is nothing). Sans isn't above using underhanded tactics in battle, as shown when he tries to manipulate Frisk into sparing him, then killing them if they yield.