Frisk/the player first meets Sans soon after exiting the Ruins into Snowdin Forest, where Sans first appears to be a silhouette which stalks the player, and to the surprise of the player, is a friendly and welcoming character. He then hides the player from his brother, Papyrus, and explains that despite being a sentry, he has no interest in capturing humans. However, he says that Papyrus does, though assures that he is not dangerous. The puzzles he sets, such as the “Monster Kidz Word Search”, are very plain and easily avoided, showing his true lack of interest in catching humans.
After random encounters with him before reaching Snowdin, his house (which he shares with Papyrus) can be accessed if the player goes on a date or hangs out with Papyrus. During this, the player can access several items around the house that demonstrate Sans’ comical nature, including a sock he refuses to put away, and his “humerus” entrance with a tromBONE. When Undyne asks Papyrus how they can afford to have such a big house in Snowdin, Papyrus says that Sans pays for it; when questioned further about it, he says it’s a mystery.
Just after entering the Waterfall, Sans offers player to hang out with them. If the player agrees, Sans takes them to Grillby’s by using a shortcut. They reach it instantly, demonstrating his capabilities of teleportation. Sans is greeted warmly, showing he’s quite popular with people. Sans tells the player that he is concerned about his brother who is being ‘pranked’ by a flower telling him motivational things (it is strongly hinted to be Flowey, as Flowey later says that Papyrus is in his fan club). He passes it off as an Echo Flower. Before leaving, he pranks the player saying that they should pay for the food, presenting an unreasonably high price for it. Regardless of if the player says yes or no, he laughs and asks it to be put on his tab. Sans is encountered again in the Waterfall, letting the player use a prank red-eye telescope.
The player can then meet him outside MTT Hotel, where Sans offers yet another option to hang out, if agreed, Sans takes the player (through another ‘shortcut’) to MTT Restaurant. There, Sans expresses his support for the protagonist. He mentions that he has been friends with a woman for a long time, meeting her first while practicing ‘knock, knock’ jokes at the gate of the ruins. She enjoyed his bad jokes and they became friends, although never meeting each other until in the Pacifist Route’s ending. Sans then told the player that the woman made a request: to watch over and protect any human who came through the door. He goes on to say that while he never makes promises, he could not say no to someone who enjoys bad jokes as much as him. The player is warned that if not for that promise, they would have been “dead where they stood”, making it clear that Sans would have killed them himself. Coming off the threat as a “joke”, he changes the subject to how he must have done a good job protecting the player (from his perspective, the protagonist hasn’t died a single time), telling them that someone really cares about them before leaving.
At the end, when passing through the last corridor, the player meets Sans, where he says he will judge the player for every thing they have done, showing how Sans has been observing player since the start. He mentions the true meaning of EXP (execution points) and LOVE (level of violence). He cuts his speech short to reveal himself and talk normally, telling the player that they never gained any LOVE. He goes on about how he appreciates the player for having kindness in their heart, telling the player that the Monsters’ fate is in their hands: let Asgore take the player’s soul for their freedom, or take Asgore’s soul for the player’s escape. He tells the player that he believes their Determination will help them do the right thing. He then leaves.
Loading the game and returning to the last corridor will result in Sans telling the player that he suspects they have some sort of time travel ability. He whispers a secret codeword to the player and asks them to ‘come back’ and tell him the codeword. Loading the game and returning two more times, Sans gives the player the key to his room in Snowdin, telling them that it’s time they learned the truth. In Snowdin, Sans’s bedroom contains the silver key, which unlocks Sans’ workshop behind the house. The room has a different decor to the rest of the house and it contains blueprints, photographs, a badge and a broken machine.
He can be seen coming during the cutscene while preventing the battle with Asgore, saying “hey guys, what’s up?” Toriel recognizes the voice, and they meet face-to-face. Toriel recognizes Papyrus as his brother, saying that Sans has told her a lot about him.
During the final battle against Asriel, Sans in his lost soul form, says things like “just give up, i did” (possibly because he is aware of his inability to manipulate the time loops and SAVEs), “why even try”, and “we’ll never see ’em again”. During this, if the player does a bad joke with Papyrus’ soul, he hates it, but Sans’s soul likes it. After Asriel is defeated, they destroy the Barrier, and the cast returns to the surface. Papyrus leaves to make a ‘good first impression’. Sans says someone’s got to keep him away from trouble, and runs off to ensure that it’s not him, leaving Undyne to do it. In the credits, Sans is seeing riding a tricycle on a highway, while his brother races along in a car. Papyrus seems angry when Sans overtakes him.