"Obsidian is a henchman working for Quakemaster. He appears to resemble a jester, with an unexpressive mask-like face and hair with four points that end in playing card symbols. Beneath his mask, Obsidian is pretending to be a normal Crystal Kingdom resident turned to stone by Quakemaster's curse, sharing the end of the fate Sir Carrot is currently going through. However, in the continued illustrated script comic, he admitted to using an illusion to appear that way so as not to upset the Nightmare Knight. In reality, Obsidian was originally a Crystal Kingdom knight who was sent to kill Quakemaster, but who instead saved his life, staying with him while he healed and subsequently growing closer. Quakemaster and Obsidian fell in love, but since it would not be aligned with the Nightmare Knight's plan for a disaster master to fall in love with a regular citizen, Quakemaster has Obsidian masquerade as his servant so they can remain together. He seems to have been extorting money out of King Ruby for some time." -Cucumber Quest Wiki

I don't have a specific voice headcanon for Obsidian, so go nuts!

NOTE: Obsidian is MLM, so I would prefer that a MLM individual audition for the role. Anybody is welcome to audition for him, though!