Sans the Skeleton

Sans in the Judgement Hall

Sans is the older brother of Papyrus. It appears at the beginning of the Snowdin Forest, located on the outskirts of the village of Snowdin, giving a small scare to the fallen human due to the sounds it makes when it approaches it. Depending on the Frisk's choices, he can be both a friendly character and an ally, as well as a heroic antagonist.
Sans is a skeleton of short stature and large bones, dressed in a coat with a fur hood, black pants with a white line on the sides, a chain mail and black boots and is always carrying a bone larger than him, pretending to be a staff or a spear without a tip.

He is always seen with a smile and only moves the corners of his mouth a little. He has white pupils that tend to disappear when he speaks in a serious tone. A bright and intermittent iris of blue and yellow colors appears in his left eye during the battle against him on the Genocidal Route.

Sans makes his scythe appear.

He has obtained the knowledge of the multiple lines of time and the function of the SAVE, apparently through his scientific investigations. He seems to have knowledge of when the Frisk loads a SAVE file, although he can not use the SAVE function himself. However, he does not seem to be able to remember what happened when doing a RESET, although he can recognize when the player has made one. Apparently his knowledge of the situation and his innate perception allows him to recognize when the fallen human has been behaving in an unusual way.

The dagger with which he killed
Chara on the Genocide Route.

Similarly, it has shown the ability to move incredibly quickly and/or teleport with the frequent use of "shortcuts" to get anywhere. During the battle on the "Genocide Route", he can shoot what appears to be a type of lightning using a weapon called "Gaster Blaster". The Gaster Blaster have similarities with the dragons, only that here are skeletons, when the invocations of the Gaster Blaster comes to frequent during the fight, these take the form of a Hydra, the monster with multiple heads and the level of its rays are each more deadly times.

Sans is shown with a very relaxed personality, sleeping at work or taking breaks frequently. He enjoys constantly making bad jokes related to skeletons, to the displeasure of his brother. It is good and comforting in nature, but it becomes sinisterly serious in abrupt moments (particularly when angry)

Sans is a user of magic, so he uses the Wizard class, he has a long bone that he uses as a weapon, pretending to be a magician's staff, he has the skills of "invoking" this allows him to materialize skeleton dragon heads that shoot rays of its jaws and when that invocation reaches the limit it could become a Hydra.
Much is unknown about his past and the research he usually does. Practice alchemy and study of the stars. Usually it is sluggish in front of every situation but always expectant before the movements of the fallen human.
If the Genocide Route is executed and Papyrus dies, Sans will remove the "the Clan of the Friendly Bones" badge and will be known by the remaining monsters as "Deserter" and will only follow the human without caring until his reach the judgment hall.