Before his gospel death
At first, Sans looked exactly the same. However, he was usually seen wearing a tuxedo, white undershirt and black tie.
Descending to Hell
He looked similar, however, he was beginning to turn to stone.
After getting "new clothes"
He now looks like Frisk, however, he now has yellow eyes, and made mostly out of stone. He wears no clothes except for pants, and has large horns on his head, as well as feathery wings.
Sans had a horrible injury, and was trusted in the hands of Doctor Alphys. However, instead of trying to save him, she tried to not him unconscious long enough to be legally declared dead. However, he escaped his watery death inside a coffin, managed to get away from her army of surgeons, but...died. He had ascended to Heaven, only to be banished to Hell. Many years later, a human child fell into Mt. Ebott. And the Emperor found new clothes.
Flowey is his trusted back-up singer.
"Finders keepers, losers weepers." he said as he took control of their body, returning to rule Hell with the power of a human soul.
His other back-up singer.
His backup singer and brother, The Emperor lost all respect for him after his descent to Hell.
- Truth be told, he never was yours.
- His song can get stuck in your head.