Null is the result of the restorage of the remnants of Sans' data after he was erased by a data wipe. He is considered part of the game, and Papyrus claims "noticing Sans' essence still somewhere in the universe" (presumably being Null himself).
Undertale's game data wipe
It was a normal day in the universe. Everyone was enjoying their lives. But then a tragedy happened. The timeline got corrupted, and a huge data wipe occurred. This heavily affected Sans, partially removing him. But some leftovers were still there, and Null was created.
Gaster eventually finds the exit of the void, and finds out about Null. He decides to gather several pieces of data from other sources, and creates some own. Those are "injected" (or installed) into Null, which he becomes fully existent. He is still not the original Sans, but he shrugs it off and continues his life. Since having data from other sources, he is now able to become a spectator and leave/enter his universe on a whim. Since then, he has gained feelings and emotions.
He appears to be Sans' silhouette, being purely black and having startling red eyes. It is unknown if he has actually a hoodie or not. A part of his sweater/jacket is still visible, but it is a much darker color.
He often watches what is happening in his universe. He is known to be void-like inside: he is lacking a SOUL, and therefore lacking emotions. He still can produce thoughts. His devoid emotional state causes him to be fully immune to any and all emotional pains.
Inside the game, he's known to be without battle mode data, as well as stats. If a battle starts and the CHECK button is used, it will say "null - ATK DF". Seconds later, the battle will end automatically (at the moment Null tries to attack.)
- His story is inspired by Error!Tale.
- His name is inspired by the Minecraft creepypasta with the same name.
- He, along with his universe's Chara, seem to be the only known ones affected by the wipe.