Lt. Ethan Warren was a soldier during World War II in Wolverine's same platoon. He was caught by Japanese soldiers along with Logan and was put in a cell, from where the two of them easily escape. During their run in the woods, Warren and Logan met a Japanese woman, and Warren declared he wanted to kill her, only to be threatened by his companion, who told him that, if he shot her, he would have met the same fate. While Logan stayed with the woman, Atsuko, finding she didn't have any intent of arming them, Warren departed.
After a while, Ethan Warren comes back and, after shooting Logan, tries to rape Atsuko. The woman tries to defend herself by stabbing the soldier with a sword, but Warren stabs her in return with his bayonette, killing her. Logan wakes up in that moment and attacks Warren, and during the fight the two of them discovers the other to be a mutant, both of them immortals. The fight goes on, with one incapable of killing the other for good, but suddenly a plane drops something nearby: Hiroshima's atomic bomb. The two mutants are invested by the explosion and are parted.
Ethan Warren survives the H Bomb, but, not having Logan's healing factor, lives as a flaming skeleton. In present day, after M-Day, Warren manages to find Wolverine in Japan, and, without even revealing himself, attacks his former enemy. Logan, after recognizing him, tells Warren he doesn't want to kill him, but Warren replies opening Wolverine's chest, ripping his heart out and eating it. By eating Wolverine's heart, Warren regains human appearance, and is about to leave. Logan, anyway, doesn't give up, even without his heart, and stands up to fight again. Caught by surprise, Warren attempts to fire a searing blast to Wolverine, not even hurting him, and then pleads him for mercy. Wolverine shows none, and kills Lt. Ethan Warren for good.