Sans and Papyrus were taken from the suspension tubes together but both had very different reactions to the world they found themselves in. Sans considered Gaster to be a threat from the start.
Post the brothers meeting, they were kept together to learn off one another. Sans has suffered more than Papyrus has at Gaster's hands. He has almost died on atleast one occasion.
Later into the series, the brothers begin to see glimpses into the future. They are seen as a source of hope for the brothers as they are not in the Lab but the images they see are usually from Frisk's Genocide run, it looks clever since its where they are killed. ( Papyrus in the end of snowdin, and Sans in the judgement hall. )
Sans' body appearance is similar to classic. The only difference in his body is that there is a handplate on his right hand, Its signed '1-S' the reason why he is left-handed is because looking at his right hand makes him think of times being tortured by Gaster with his brother.
He also didn't wear his normal outfit since they are not freed yet.
Sans' personality is similar to Classic Sans. Like Classic he is smarter than Papyrus, but he is physically much weaker and has less stamina. They are generally considered to be twins but as Sans is more fragile, he may have been the first attempt. According to a recent piece by Zarla, the brothers would refer to one another as 'big brother' in Japanese but using different terms.
His pupils change color depending on the mood, if he is happy, it change into Green, if he is fearful, it change into Purple, and so on. However, he sometimes glow his blue eye, His left eye was broken on later on the story.
Sans can heal, like Papyrus, but is much less proficient at it than his brother. When he tries to heal Papyrus, he is never capable of doing it fully.
Sans was very attached to Papyrus but their bond can be broken ( Like Sans breaking Papyrus' feelings because Gaster can't accept mercy to them. ).
While still having most of Classic's abilities, the bone attacks and Gaster Blasters, he lacks one key skill. The ability to bypass invincibility frames is missing from S-1's skill set which means that he is next to useless in standard combat. Despite this, has done major damage to Gaster before to the point of almost killing him.