Sans

Sans is a major character in Undertale.

One of the first characters the player will meet outside of the Ruins, Sans will initially toy with them through horror style surprises like splintering a heavy branch in the path without being seen. When the player reaches the gate meant to block off the path to the Ruins, Sans will approach them from behind as a shadowy figure to greet them with a handshake prank. How the rest of the meeting plays out will be affected not only by which route the player is taking but also by how many times they have already played the game before.

In the Genocide route, Sans will note how the player barely responds in anyway to the "whoopee cushion in the hand" trick, making a side remark of how a "lady" really knows how to "pick 'em." Sans will then introduce himself and say that while he is supposed to be on the lookout for humans, he doesn't care about capturing anyone. However, he describes his brother Papyrus as being a human hunting fanatic, and noticing his approach, says he has a plan to help you get past him. Taking you through the gate, Sans will tell the player to hide behind a nearby lamp but they will just stand in place. Even so, Papyrus won't even acknowledge the player's presence and leave as quickly as he came. Just before the player leaves the area, Sans will tell them that Papyrus has always wanted to actually see a human, and it would help if the player continued to "pretend to be one."

Later on, the player will next encounter Sans past the Box Road where Papyrus will ask him about when "the human" will show up. Sans will suggest that he look in the direction the player just came from, but Papyrus will have no idea what he's supposed to be looking at until Sans helps him with the process of elimination. After Papyrus "warns" the player of what will happen if they proceed onward, Sans will once again point out that the player didn't respond in any way throughout it all and proceeds to follow his brother. The player will see Sans next at the Invisible Wall Maze puzzle, where Papyrus will accuse him of "napping" all night. Noticing the player's approach, Papyrus will attempt to explain the puzzle only for them to walk on through before it can be activated. If the player speaks with Sans before moving on from there, he'll say that it make Papyrus feel better if the player went along with the puzzles. Passing through the next area, the player will encounter the brothers again at Sans' "obstacle," which is a cross-word puzzle on a piece of paper lying on the ground. Ignoring the puzzle, Papyrus will quickly point out that it did nothing and Sans will reply that he should have picked "Junior Jumble" instead, which is Papyrus' favorite puzzle. Talking with Sans before moving on from there will reveal that he actually prefers "funny pages" over word searches. After passing by some of Papyrus' unactivated puzzles, the player will come to the Colored Tile puzzle and proceed to walk through it before Papyrus can explain it. Sans will suggest that maybe the player just doesn't like these kinds of things, and after talking about it, Papyrus will ask for the player's opinion only to receive no response at all. After Papyrus leaves, the player can talk with Sans before moving on, wherein he will say that puzzles could be fun if you actually gave them a try.

Just as the player is crossing the bridge into Snowdin, Sans and Papyrus will appear at the other end to stop you with a far more deadly trap. After Papyrus declares that he will activate the trap, Sans will remark on how it "doesn't look very activated" prompting Papyrus to admit that the player will just walk through it before it'll activate. Just when Papyrus is about to admit that maybe he shouldn't have put so much effort into his puzzles and be lazy like Sans, Sans will joke about how "ridiculous" that is and convince Papyrus that he just had "the wrong audience" for his puzzles. Should the player speak with Sans before moving on to Snowdin, he will say that you are going to end up fighting Papyrus and warn that if you continue what you have been doing "you're going to have a bad time." Sans will then disappear right before you.

Sans will not appear again until the player reaches New Home, where he will appear in the hall leading to Asgore's garden. If the player didn't kill Papyrus earlier on, they won't have to fight Sans but he will make clear how disgusted he is with the player's actions. If the player did kill Papyrus along with everyone else, Sans will try to talk some sense into them but the player will ignore them, prompting him to fight the player saying "Sorry old lady. This is why I never make promises."