OMORI

After defeating Sweetheart, go back to her basement. You'll find a hole in the wall that was initally covered by a bucket of paint. Going into it, you find Rococo, Sweetheart's ex-lover. He tells you a lengthy story about his life and how he got into the room you entered. Interacting with him, you can commission art from him, for the price of 1000 clams. If you want to immediately get the achievement, get out of the castle, exit Sprout Mole Village and then re-enter his room. You'll get a piece of art called Untitled, an amateurish but endearing piece.

This achievement ties with Patron of the Arts, which requires you to commission all 6 pieces of art (requires 100,600 clams, so hope you saved up enough).