Ermey was a drill instructor with the Green Lantern Corps and trained such notable Lanterns like Kilowog and Tomar Re. Ermey was particularly difficult on Kilowog, intentionally mispronouncing his name and chastising him for calling his pregnant wife during basic training. Ermey was fond of training his cadets on a unique planet which was split into four very distinct climates and terrain. There the ring less cadets faced many trials such as climbing hand over hand over a pit of lava, trekking through a desert filled with sinkholes, enduring freezing waters, and fighting a massive squid under the sea. Ermey used the unique planet to train his cadets for the myriad number of worlds they may find themselves in, while serving in the Green Lantern Corps.
following a grueling day of training then cadet Kilowog asked Ermey why he pushes them so hard, and questioned why he wanted them to fail. Ermey came nose to nose with Kilowog and angrily made it clear that a lowly cadet had no right to question his commanding officer; but Ermey then grew more somber and told Kilowog that this job was more important to him then Kilowog could imagine. "Green Lanterns die so that innocents can live" said Ermey, who went on to tell Kilowog that growing old in the Corps was a privilege, and it was Ermey's job to make sure that every Lantern he trained was equipt with the skills to survive.
The cadet's were then awakened by a distress call. A group of Lanterns had been under siege by heavily armed raiders on a nearby world. These raiders had no compunctions against killing innocent civilians or Green Lanterns. The Lanterns needed immediate assistance, they were nearly out of power and were being blocked from accessing their power batteries. Ermey and his cadets were the closest Lantern available to respond. Giving the cadets back their rings, Ermey led his soldiers into battle instructing them to follow his every command.
Arriving on scene Ermey moved in to intercept a heavy canon before it could fire on the deplete Corpsmen and their civilian charges. Ermey blasted the raider stationed at the canon and lifted it in the air, intent on turning the gun against his foes. Unfortunately Ermey did not see a second canon behind him, and the Lantern Instructor was brought down with a massive blast to his back.
The cadets had lost their leader but before the raiders could ready their guns to slaughter more innocent lives, they were blasted by a barrage of fire from the canon that had killed Ermey. Kilowog had taken out the gunman and turned the raiders own canon against him. Kilowog relayed orders to Tomar Re to shield the civilians and the depowered Lanterns, before firing repeatedly on the raiders who had penetrated the Lanterns defenses. Those raiders who weren't killed by the barrage, fled, and the innocents were saved.
Kilowog turned his attention to his dying instructor. With his last breath Ermey told Kilowog that he thought there was a leader somewhere inside of him. He then raised his bloody hand to Kilowog's chest and drew a lantern symbol over the cadet's white circle. With that last gesture, Ermey made it clear that he thought Kilowog had passed basic training and was a Green Lantern.
Ermey and Kilowog's story was told on screen in the animated movie Green Lantern: Emerald Knights. Although, Ermey's name was changed to Deegan and his was appearance altered to a more alien visage, the story was nearly identical to the printed version.