666 (Three-Six) is a genetically engineered prison guard created through experimentation at the lunar Dead Leaves correctional facility. This entity possesses distinctive green skin, a tall and thin physique, and notably elongated limbs. Its primary combat weapons are arms that transform into sharp blades. An enhanced physiology grants high-speed movement, particularly in combat, enabling rapid strikes against opponents.

Alongside partner 777 (Triple-Seven), 666 enforces discipline under Warden Galactica, suppressing inmate activities and quelling prison rebellions. The character exhibits overt sadism and enjoys violence, demonstrated by actions like licking its blades during fights and mocking victims. A dynamic exists with 777, contrasting 666's lean form against 777's bulkiness; friction occurs, such as 666 admonishing 777 for praying before combat.

During a mass prison break instigated by inmates Retro and Pandy, 666 engaged Pandy in direct confrontation. This featured extensive blade combat that partially destroyed Pandy's prison uniform. 666 exhibits significant durability, dismissing conventional bullets as ineffective. Its ultimate fate remains unresolved following the destruction of the Dead Leaves facility during the climactic battle involving Galactica's transformation and a mutant baby's sacrifice.