Sans has a secret identity in a world full of musical monsters. After so many resets, he had gotten tired of his life, so he became someone else: the Masked Musician. He didn’t even have to choose a name, because the monsters did it for him. He wore a mask and a simple suit with a hood, played so many instruments that he conjures with his magic.
Piano, cello, a harp or even a spacedrum. He played them all, because he’d learned to play them from so many monsters from so many resets. He’s tired. So... tired.
Renditions, remixes and even his original works... he plays them all, trying to erase the apathy from his soul.
Trying not to let Papyrus know that it was him who stole his heart... the composing star, Musica Symphonia, whose music cries of yearning and anguish.
(Get it? Comic Sans MS?)