In the beginning was him, crying in a pod in the middle of space, witnessing his home planet blow up. When he awoke, he found himself in the middle of a desert in a sea of brown sugar which he learned was ORANGE OASIS. Even as a baby, he knew he was the sole survivor of his species, the last elf in the entire universe. He was found by a young DONUT by the name of SWEETHEART. She brought her to her family and they took him in as their own. He and SWEETHEART grew up together. The two fought frequently but SWEETHEART would always win. He became good at running and knew he had to survive to repopulate his species.

Eventually, he and SWEETHEART fell in love, became engaged, and moved into SWEETHEART'S CASTLE. But, as SWEETHEART's fan base grew, the two grew apart. When she began searching for suitors, ROCOCO objected so, when he was asleep, SWEETHEART and her servants tied him up and sealed him inside the walls of her castle and he has been wandering through the walls ever since. At some point, he decided to become an artist.

OMORI and his friends are the first living beings he had seen in years, and he begs them to buy his paintings.

ROCOCO should be flamboyant and dramatic in the delivery of his lines. 

James from the Pokémon Anime could be a good reference for his voice if you are unable to do a French accent.