They are very much the same character, but fans took the 'Sans has a hidden darker powerful side' idea and rolled with it, creating dramatic interpretations of his character. The Sans you know in a pacifist playthrough and the Sans you meet in a no-mercy playthrough have very different attitudes. Sans' magic is usually shown as glowing blue, though this isn't really present in canon. The idea of the sexy ectoplasm tongue in sexy Sans Undertale fanart circles got so popular that is spilled out to the point where basically everyone, even completely sfw artists, were on board with the idea of Sans being able to summon glowing blue ectoplasm body parts. You know that weird thing where characters get drawn with uncomfortably long and slimy sexy glowing ectoplasm monster tongues and bedroom eyes all the time? They invented that for Sans. It only shows up once in the game, when he uses his magic while he's going all-out during the genocide route final boss, but the fandom has exaggerated it to being a main feature of his. In fanart, they are often depicted with sharp teeth, and a lot of his AU selves have sharp teeth. What's interesting is that many of the monster characters have sharp teeth, but Sans and Papyrus do not in canon. He is part of the 'monster race' in Undertale. Sans has a much more detailed description on the Undertale Wiki. The Undertale fandom was known to be disproportionately horny for nearly all the characters, but Sans' position as the fan favorite saw him as the most prominent subject of that horniness. The dramatic tension of the scene skyrocketed Sans into being the fan favorite, so much so that he eventually represented Undertale in Super Smash Bros. In this battle, Sans has a far more aggressive and involved attitude than in the rest of the game and reveals a surprising amount of raw combat power. This battle is often considered the most difficult boss battle in the game. In the game's Neutral or Pacifist routes, the player will never fight him, whereas in the game's Genocide (or No Mercy) route, Sans is the final significant boss. His role in Undertale's story depends on the route taken. He has a brother, Papyrus, whose energetic and genuine personality contrasts Sans' lazy jokester personality. Sans is a skeleton, and the first character the player meets outside of the Ruins. 2.6 Fanon Splintering and Self-Shipping.

Undertale pacifist fanart