Unknown (presumed dead)
J. Yezback was a Mars-Sec security guard stationed at Mars City. He was assigned to guard the security door leading to the Administration facility. Yezback wore a standard Mars-Sec uniform and full helmet, and wielded a Machine Gun.
When the player meets J. Yezback in Doom 3, he says that you don't have clearance to enter Administration. If the player persists in speaking to him, he will become very annoyed and eventually snap, "God dammit, Marine, get outta here!"
His fate is unknown, though he was most likely killed or turned into a Z-Sec machinegunner during the demon invasion.
His behavior could be an obscure Easter egg to Half-Life's Black Mesa Announcement System. In Hazard Course of said game, if the player tries to continuously access a restricted area without approval, the announcer would eventually become annoyed and snap at the player.