Hades

As you go through the story, you'll start noticing Hades mention Orpheus more and more. Orpheus is a legendary singer who has been imprisoned by Hades. The first thing you'll need to do is to free him by purchasing his "sentence" from the House Contractor. Once you have told Hades you want to do this, the "Court Musician's Sentence" job will be available to purchase for 1 Diamond. You'll find it under the first tab, Work Orders. Unfortunately though, freeing him is not enough. Once you've freed him, he'll be sitting in a chair to the right of Hades, but he'll still refuse to sing. However, you'll now be able to give him Nectars when you return from escape runs. Give him 5 Nectars and after that he'll start to sing again.

Note: Once you've freed him and gotten him to sing, if you've purchased the Music Stand, you can start buying new songs from the House Contractor under the Court Music tab. These songs all cost Diamonds to purchase and can range from 1 to 6 Diamonds apiece. These are 100% optional and neither the Platinum nor the game itself requires you to purchase any songs, so I'd suggest you leave these at least until much later in your playthrough, because you'll definitely need your Diamonds elsewhere. (You might need to purchase Eurydice's Good Feeling song when it becomes available for the Musician and Muse  trophy, but that's unconfirmed.)