"I am SANS, caretaker of the RUINS. "
– Sans introducing himself.
He is the keeper of the ruins and a former king. Sans left the throne and moved into the ruins after his brother betrayed him. He also provides shelter for the 6 children before they leave.
Altertale Sans wears a black hooded robe with long sleeves and a dark/light blue and white side slit over shirt with the Delta Rune symbol over the robe, along with pink slippers. This Sans is taller than the original. He looks about 35, but he's really 500+.
- Toriel's personality prevails over the original Sans' personality.
- He is the elder brother of Papyrus and Gaster.
- He is the keeper of the ruins.
- He can still use bones and "Blue Heart" in battle.
- He can use telekinesis to move objects.
- He does not use Gaster Blasters.
- When AT Sans uses magical powers, both of his eyes light up blue.
- Sans no longer speaks in all lowercase.
- He has lived alone in the ruins for many years.
- He despises himself for not being strong enough to save the children.
- Despite many years of suffering, he is incredibly calm and relaxed, but sometimes it turns into a form of rudeness and savagery.
- His calmness is his protective mechanism.
- Sometimes he greatly misses his brother, although he diligently tries to hide these feelings.
- He often recalls his late brother, Gaster, and the fallen children.
- He has strong paternal instincts.
- Has the option to be called "big brother" instead of "dad".
- He loves puns, frogs, and home-made hot pockets. He is only occasionally lazy.
- He gives frisk the choice of either a pepporoni or ham and cheese hot pocket.
- Forgives Papyrus at the end of a Pacifist Route, but the restoration of good relations among the brothers takes a long time.
- At the end of a Pacifist Route, Sans becomes a therapist.
- Used to race frogs with Napstablook.
- Probably hundreds of years old, but looks about 35.
- Good hider.
- Is a boss monster.
Sans and Toriel are mutual love interests.
Sans and Papyrus were close, but Papyrus' decision to kill human children drove his brother away. This is how Sans came to live in the ruins.