There's a lot to say about this dude, so I'll try to sum it up quick. Sans is considered a lazy guy who likes sci-fi comics, eats only junkfood, doesn't clean up after himself, plays constant pranks & jokes just to rile people up & gain amusement from it - & seems to know everybody. He's cold, but friendly, just like the snowy town he lives in & has a deep but chill voice. Said voice can become even more chilly if Frisk does questionable things. Only really seems to care about his brother & is keeping a watchful eye on Frisk. He seems to know more about things than he lets on & comes off as kinda mysterious to the fandom because of this.