Shield Sheldon is a Reploid based on a clam who appears as one of the Nightmare Investigators in Mega Man X6. He was originally created by Gate to serve as a bodyguard for VIPs, but a Reploid scientist under his protection turned Maverick and Sheldon was forced to eliminate him. When the Maverick Hunters arrived on the scene, they reached the false conclusion that Sheldon had murdered the scientist and had gone Maverick himself. Overcome with shame, Sheldon took his own life. He would be revived by Gate years later to aid in the investigation of the Nightmare Phenomenon.
- Guard Shell - Shield Sheldon's shell is virtually indestructible and capable of withstanding any attack. He can withdraw into his shell and bounce around the boss-room. Both X and Zero can copy Sheldon's data to gain their own version of the Guard Shell: an energy shield that can reflect certain projectiles back to their source.
- Shield Throw - Sheldon can unclip one of his shell halves and throw it like a boomerang.
- Shield Clone - Sheldon creates an indestructible energy-based copy of himself that mirrors his actions.
Shield Sheldon's shell cannot be penetrated by any weapon, but his body is weak and is particularly vulnerable to hard impacts from attacks such as the Metal Anchor and the Rakukojin.