A monster can have any kind of muse.
A Muse, however, can also be another monster.
When a monster sings or plays music that is inspired by their Muse, the Muse’s soul feels the music like an ‘offering’. Their soul glows, and they would play the Musician’s music right then and there, like their souls are music players. The most proficient Muse can feel the emotion and theme behind the music, they can hear the story.
Papyrus has lived alone for most of his life, with songs of encouragement spilling from his soul. He never knew any relative other than his late father, Gaster. The doctor had been mostly busy with scientific discoveries until his untimely demise, like the rest of the previous Royal Scientists. He left Papyrus an unimaginable amount of wealth.
It’s a wonder why Papyrus has the most beautiful music coming from his soul. It never stops playing, keeping him forever happy and inspired. The power of their magic shone much more brightly than ordinary monsters’ when they played for him.
He just can’t help but wonder about the monster who keeps him an inspiration for so long. Most Muse lose the song of monsters all the time, but after years and years of never-ending arrangements of songs, Papyrus doubted it would change.
How could they play music all the time? Do they ever get tired? Where do they even get the inspiration to keep playing new songs all the time? He’s already lost count after several hundreds, but Gaster’s machine hadn’t.
When Gaster died, he had left a box that contained clues to the identity of the monster playing eternally for Papyrus -- an extremely powerful monster, based on Gaster’s notes. It says to find the monster and save them before Papyrus bonded with anyone.