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.