You had fallen underground, after meeting Yin Flowey, and he helped you out of the ruins, you went to the city of Snowdin that Yin Flowey indicted.

Y!Sans: Hey buddy! Wait one moment!

Someone caught your eye, and as you turned around you noticed a short skeleton running towards you

Y!Sans: Hey! I'm Yin Sans! And you? You seem to be..new here huh? Pleasure!

he extends his hand