Who is Sans? How Undertale’s secret boss became an indie icon

Despite playing only a small role in Undertale, Sans is now one of the most famous characters in all of gaming. 

From his iconic theme song to his unmistakable voice tone, it seems that gamers just can’t escape from Sans. Undertale’s most fascinating support character is a constant fixture throughout the game, offering advice and warnings to the player. His mysterious nature and laid-back attitude make him a core part of almost every Undertale fan theory. For fans of the series who want a refresher or players just now picking up Undertale and Deltarune, here’s everything there is to know about Sans.

Undertale’s most iconic character Sans is surpsingly humble

Sans is one of many supporting cast members in Undertale. He appears at various points throughout the game to guide the player or to warn them about the consequences of their actions. Sans wears an iconic blue hoodie and black athletic shorts. While his eyes are usually pitch black, one of them can glow blue under certain circumstances. Sans does not play a clear role in the underground monster society. 

He and Papyrus are brothers, and Sans will strongly react to the way that the player interacts with Papyrus. Sans appears in almost every major area before or after key events. His exact role varies depending on the game route that player chooses. In pacifist and neutral runs, Sans will act as a supporting character. In genocide runs, Sans switches to a primary antagonist who attempts to stop the player at multiple points, either by force or through appealing to their emotions.

How to fight Sans in Undertale

Sans’ boss fight in Undertale is locked to the very end of a genocide run, and it’s by far the most difficult battle in the entire game.

While normally a friendly face, Sans becomes a direct antagonist to the player in the genocide run of Undertale. This playthrough requires the player to kill every single monster they see, including random overworld encounters. All named characters like Toriel, Papyrus, Undyne, and more must be defeated in their story battles. A number of other tasks must also be completed in order to fight Sans in Undertale.

Sans will meet the player in the final corridor just after Asgore. He will confront the player and threaten them before initiating the battle. The following fight is extremely difficult and has several gimmicks. Instead of dealing damage in chunks, Sans deals just one damage several times per second that slowly ticks down the player’s health. His health pool is very large and the fight challenges the player’s endurance. 

Players who defeat Sans are immediately leveled up to the level cap, which is useless since he is the final fight of the game. His dialogue will also change depending on if the player has previously defeated him.

What is Sans lore in Undertale?

Sans’ lore is that he is the only underground monster aware of the player character’s powers. He acknowledges the power of determination as a way to reset time back and forth. It is implied that, while Sans has awareness of the ability, he is unable to use it himself. This doesn’t really matter in the pacifist and neutral runs of the game, but it has a significant effect on the genocide route.

When Sans challenges the player at the end of a genocide run, he does so knowing that no matter how many times he wins, the player can always reset time and try again. 

Sans is also loosely connected to another mysterious Undertale character, W. D. Gaster. Their exact relationship is not known, but Sans owns a hidden machine that appears to be made by Gaster and uses weapons internally called “gasterblasters.” This lack of information has made Sans a frequent subject of Undertale fan theories and content.