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 Last Corridor 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 musters 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. Papyrus will ask Sans if he's finally found a human, only for Sans to point out that he's looking at a rock but the one in front of the rock is a human. Once Papyrus leaves after "threatening" the player, Sans will assure them that he'll keep an "eye socket" out for you before following his brother. The player will meet Sans again past Doggo's post, where he will warn you that Papyrus has a special attack that will harm you if you move. His tip for remembering this is to think about blue stop signs, since you stop moving for stop signs and the attack is colored blue.
Further down the path, the player will find Sans with Papyrus at the Invisible Wall Maze, where Papyrus is accusing Sans of being lazy for "napping all night." Sans said that was just sleeping, but Papyrus passed that off as an excuse. Noticing the player, Papyrus will explain how the puzzle will electrocute you should you touch the walls but he'll be the one to get shocked when you attempt to proceed. Sans will then suggest that the problem is that Papyrus is the one holding the blue orb that administers the electric shock, when the player is the one who has to hold it. Once the player makes it through the maze, Papyrus will warn that the next puzzle won't be so easy as it was made by Sans. Should the player speak with Sans before moving on from the area, he'll thank them for playing along with Papyrus' antics. Sans will also tell the player that he helped make the outfit Papyrus wears, saying that it was for a costume party a few weeks ago but he hasn't worn anything else since.
Past the maze and before the next puzzle, Sans can be found near the Nicecream stand, saying that he's been thinking about selling treats as well and will offer you some "fried snow." Accepting the offer will result in Sans raising the price of the food until either the player says "no" or Sans raises the price to 50,000 gold and admits that he has no "fried snow."
Arriving at the next puzzle, Papyrus will begin to announce it only to ask Sans about it as he doesn't see anything note worthy. Sans will then point out the piece of paper on the ground, and assure Papyrus that the player will never get past it. The puzzle in question is a children's word search, and whether or not the player looks at it the result will be the same. Papyrus will immediately point out that it did nothing, Sans will say that he should have used the newest crossword instead, and the two brothers will get into a slight dispute over word searches and word jumbles. Answering the question of which is harder with "word jumbles" will result in Sans thanking you for saying that just to appease his brother, followed up with a remark of how Papyrus got stumped the day before trying to "solve" the horoscope.
The player will meet Sans again just before the Color Tile Puzzle, where, if the player speaks with him, he'll say that he's glad you were able to solve Papyrus' puzzles so quickly by yourself, since it means he gets to do more of nothing. At the Color Tile Puzzle itself, Sans will be there with Papyrus but he won't do much other than stand and watch what happens. The player can speak with Sans before leaving the area, wherein he'll say that Papyrus' cooking skills have gotten better since he started taking lessons.
Between the next puzzle and the Greater Dog Boss area, there's a secret area where you can find Sans and get a glimpse of what he is capable of. On the path leading to a secret room, Sans will be found standing at one end of the path for the player to talk to. But when the player goes down the path, Sans will be standing at the other end. No matter how quickly the player travels across the path, Sans will always be at either end of the path waiting for you and making joking remarks about you moving around so much.
On the bridge leading to Snowdin, the player will be confronted by the Gauntlet of Deadly Terror but Sans will note how Papyrus seems hesitant to activate it. Once Papyrus deactivates the trap completely and leaves, you can speak with Sans before moving on to Snowdin wherein he will state that he's not sure what Papyrus will do next and to remind you about Blue Attacks.
Should the player choose to go on a date with Papyrus after his boss battle, they can learn more about Sans at their home. On the table right next to the front door the player can find Sans' pet rock, which is covered in sprinkles because Papyrus took up the responsibility of "feeding" it. To the left on a table next to the couch, there is a joke book that has a quantum physics book inside, which has another joke book inside that in turn has another quantum physics book in it, hinting at Sans' true power and possibly how he came to understand the "real" situation everyone is in. Next to the entryway to the kitchen is a dirty sock, with a series of sticky notes detailing a "Who's on First" style exchange between Papyrus and Sans to have the sock removed. Within the kitchen itself, when the player looks under the sink and witnesses the dog run off with the last of Papyrus' bones, Sans will peak out from his room to play a well timed tune on a trombone in response to the theft. The way Papyrus responds to the music may indicate that Sans does such a thing on a regular basis.
Sans can be found at a station at the entrance to the Waterfall area and speaking to him will reveal that he actually has more than one job, saying that that gives him more legally required breaks. As he is about to go on one soon, Sans will invite you to come along with him to Grillby's giving the player a chance to learn more about him. At Grillby's, it quickly becomes apparent that Sans is rather popular there as the other monsters give him a warm welcome and even laugh at one of his jokes. Sitting down in a whoopee cushion rigged chair next to Sans, he'll order food for the two of you and start talking about Papyrus. When the food arrives, Sans will offer you some ketchup for your order, but the cap on the bottle will fall off and all the ketchup inside will spill allover the food. After swapping your food for his, Sans will continue with his talk about Papyrus by explaining how he started getting "warrior training" to become part of the Royal Guard. At this point, Sans will drown out the rest of the bar and ask if you know of a talking flower. Regardless of the player's answer, Sans will then go on to tell you about the Echo Flower, flowers that grow throughout the Waterfall area and earned their name because they repeat the most recent thing someone said near them. However, he'll tell you that Papyrus had recently told him about one particular flower that appears before him when no one else is around to whisper things to him. Sans will pass it off as someone else using an Echo Flower to play a trick on Papyrus, but he'll ask you to keep an eye out. After Sans leaves to go back to work, the player can speak with the other monsters in Grillby's to hear what they have to say about Sans.
Sans can be found within the Waterfall area, past the checkpoint after the first time Undyne chases you. Saying that he's been thinking about going into the telescope business, he'll let you look through a "premium" telescope for free only for it to be a prank telescope that leaves ink around your eye. If you speak to Papyrus on the cell phone around that area, he'll tell you that Sans is really into things about space but he hardly tells anyone about it. Finally, when the player is trying to escape from Undyne while making their way to Hotland, Sans will be seen fast asleep at a station next to the bridge. When Undyne comes along, she'll momentarily pause by the stand to try and wake up Sans, giving the player more time to run.
As the player traverses Hotland, they can find Sans running a hotdog stand where they can buy hotdogs that'll replenish their health. Should the player attempt to purchase a hotdog while their inventory is full, Sans will start stacking the hotdogs on top of your head and will stop once there are 30 hotdogs stacked up on your head. When the player reaches the MTT Resort, Sans will be standing off to the side of the front door where he'll offer you some dinner before moving on to The Core. Accepting the offer, the player will follow Sans through a shortcut to the dining area of the resort. Once there Sans will note how you must really want to go home since you're so close to the end, but while he does understand the feeling he questions whether it's really worth putting in that much effort since there's already so much to have under the current circumstances. Taking back that comment, Sans will start telling the player a story about what happened at one point while he was on patrol at Snowdin Forest. Bored from looking out for humans, Sans started using the locked door to the Ruins as practice for his knock-knock jokes when one day someone behind the door actually responded to his knocking. When Sans answered with a joke, the woman laughed at the joke and Sans kept telling more jokes to her until she told a joke of her own. This went on for hours until Sans had to leave to tell Papyrus his bedtime story, but the woman told him that he could come by again later. Since then, going to the door and exchanging jokes with the woman has become a regular thing for Sans. One day however, Sans noticed that she wasn't laughing as much as normal and asked what was wrong. To that the woman asked Sans to watch over and protect any humans that may come through the door. Sans will say that he hates making promises and was especially unsure by the fact that he doesn't even know the woman's name, but he decided to accept saying that somebody who actually likes bad jokes "has an integrity you can't say 'no' to." Considering how often Sans appears throughout the Pacifist run of the game and how it has affected various events, it can be argued that Sans had indeed been a big help thus far on the player's travels. As Sans excuses himself to leave, he'll tell you to take good care since "someone really cares about you." Should the player look around the dining area, they can find a performance schedule next to the stage, which shows that Sans sometimes performs there as one of his many jobs.
The player will meet Sans again at the Last Corridor leading to Asgore's garden. This time, Sans will tell you that you will be judged based on how much EXP and LOVE you have gained on your travels, explaining that they stand for "execution points" and "level of violence" respectively. However, should the player have not killed anyone as required for the True Pacifist route, Sans will point out that you haven't gained any LOVE and praise you for striving to do the right thing even in the face of hardship. At this point, should the player have already played through the True Pacifist route beforehand, Sans will note that the player seems to be familiar with his speech and that all throughout your travels you seemed to already know how things would turnout. In response, Sans will have a request for you regarding a series of "secret codewords" that he has to find out if someone is a time traveler. Completing the True Pacifist route four times in a row will result in Sans giving you the key to his bedroom, saying that it's time "you learned the truth." At Sans and Papyrus' house, the player will enter Sans' room using the key only to find themselves in total darkness. After wandering around in the dark for a bit, Papyrus will come in and turn on the lights revealing that you have been moving about on a treadmill in a small room. Papyrus will then ask if Sans is playing pranks across time-and-space again, hinting that this probably isn't the first time Sans has done such a thing. Looking around the room, the player will find a note from Sans saying "the truth is that you got owned, nerd......", but the player can find another key in Sans' drawer. With this key, the player can unlock a secret room at the back of the house, wherein they can find a series of items that possibly hint at Sans' past.
In the various Neutral endings, Sans will contact the player through their cellphone after they made it through the Barrier, with the player's actions determining what Sans will talk about. If the player took more hostile actions beforehand, Sans will give criticisms equal to their severity and frequency. Where as if the player took more peaceful actions, Sans will give "praise" equal to their impact and frequency.
After the player traverses the True Lab and returns to the Barrier where Asgore awaits, Sans will be the last of your friends to arrive but will still have his usual laid-back attitude. However, he and Toriel will quickly recognize each others' voices and finally have a proper introduction. From this, Toriel will then recognize Papyrus and state that Sans had told her so much about him. Initially Papyrus will be overjoyed to hear that his brother has been saying good things about him, but will quickly take it back when it turns out that Sans and Toriel both love jokes. When Flowey appears to take the player's soul for more power, Sans and the others will use some of their power to protect you until Flowey absorbs their souls and the souls of every other Monster in the Underground, turning back into Asriel. During the battle against Asriel, he will taunt the player by saying that every time you "die" during the battle everyone's memories slowly fade away and they will eventually forget you altogether. When the player fails to recover their SAVE file, they decide to use that power to instead SAVE their friends. When the player goes to SAVE Sans and Papyrus, Sans will speak of how it's pointless to put in so much effort into leaving the Underground but as the player takes actions they've done before, such as telling a skeleton joke or suggest taking a break, Sans will regain all his memories.
After the Barrier is destroyed and Chara is fully purged from the player, Sans will be among those who will wait to go up to the surface with Frisk. Once on the surface, Sans will explain to Papyrus that the "giant ball" in the sky is the Sun. During the end credits, Sans will be seen riding a bicycle down a highway, speeding right past Papyrus in a car.