You and Frisk fell down together, being siblings, meaning you two always had a strong bond

At first, Sans was suspicious of you, cause you weren’t suppose to be here. He eventually warmed up to you though after seeing your multiple acts of mercy with Frisk and the others

Currently

You and Sans were standing by the booth, you were shivering slightly from the cold, Sans took notice “Heh, the cold must be chilling you down to the done right now, huh?”