Commander Steven Mullen was a Starfleet officer, and first officer of the USS Essex, under the command of Captain Bryce Shumar, in the 22nd century. Along with the entirety of the Essex crew, Mullen was killed when the Essex was destroyed at a moon of Mab-Bu VI upon being attacked by a group of disembodied criminals from a star system called Ux-Mal, who were attempting to take control of the crews' bodies and use them to escape the moon, where they had been imprisoned five centuries earlier. The Essex, however, was unable to elude the moon's electromagnetic storms, and crashed on the surface.