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."

Through out the fight, Sans will continually break the "rules" of the game and the Fourth Wall of Video Games in general. At the very start of the fight, Sans will attack first and with one of his most powerful attacks, as opposed to all the preceding fights in the game where in the player attacked first and their opponent would save their strongest attack for later. Should the player survive this attack on their first attempt, Sans will remark that he wondered why "people never use their strongest attack first." He will then proceed to dodge every attack from the player regardless of their accuracy, with Sans stating that he's not going to just "stand there and take it." Whenever the player dies, Sans will keep count and tell you how many times you have died when you come to try again, until you've died so many times that Sans decides to stop keeping track. As the fight progresses, Sans will discuss various topics relating to the plot of the game; a massive anomaly within the time space continuum throwing different timelines into chaos until "everything ends," how the player has played a part in it, how Sans gave up on trying to reach the surface because he knew that everything would just be reset with no one having any memory of it all, and that he is only now acting because of what will happen if he doesn't stop you here. After a few attempts at attacking Sans, he'll try again to talk some sense into the player. This time, part of what Sans will say may allude to how the player most likely had played through the Pacifist route beforehand, asking if they remember a time when they met and became friends. Should the player choose to continue fighting, Sans will further break the "rules" by switching in and out of different attacks in one turn, and even sending some of his attacks into the player's combat menu. After several attacks between the two of you, Sans will use his "Special Attack" wherein he will just do nothing for the rest of his turn. Sans will elaborate that he knows that, eventually, the player will be able to kill him on one of their "turns" so he's not going to let them have another turn. At this point, the only way for the player to beat Sans is by breaking the "rules" themself. Eventually Sans will fall asleep while doing nothing, at which point the player can push the action box over to the left and down over to the "FIGHT" option to attack. Initially Sans will dodge the attack like before, but halfway through Sans' response the player will attack a second time, dealing a fatal blow to Sans. After telling the player "Don't say I didn't warn you", Sans muster up the last of his strength to go to Grillby's. Off screen, Sans will ask Papyrus if he wants anything before turning to dust.

In the Neutral route, Sans will give different speeches and evaluations of the player based on their actions leading up to coming to New Home, as well as how many times that the player has actually played through the game before, remarking on how you act as though you've heard his speech already.

In the Pacifist route, Sans doesn't make any remark about the player's reaction to the trick and casually introduces himself. Noticing Papyrus' approach, Sans will enact his plan of getting the player past him by having them hide behind a lamp, of which they will comply to. Sans will then casually greet Papyrus upon his arrival, but he will immediately come to the point that Sans hasn't re-calibrated his puzzles in days. Sans will then use reverse psychology to keep the player hidden, addressing the lamp to encourage Papyrus to pay no attention to it. After Papyrus talks about how he would soon get all that he deserves, Sans will start telling skeleton jokes that Papyrus claims to hate despite smiling at them, ultimately storming off. When the player comes out of hiding and is about to leave the area, Sans will ask for you to show yourself the next time they meet Papyrus, saying that Papyrus has been kind of down recently and might cheer up from seeing an actual human. Before leaving himself, Sans will assure the player that Papyrus isn't actually dangerous, "even when he tries to be."

Past the Box Road, Sans will be seen talking with Papyrus and will notice your approach just as Papyrus was about to say something regarding Undyne.