Little is known about Ellis except that he joined the military and was put in Captain Regan's unit, presumably fairly recently given his low rank. He was stationed in the Denver Quarantine Zone before going on a rescue mission with his unit. During his Captain's birthday party, he celebrated with the rest of the squad, posing in a picture with them, which was then gifted to Regan stating she was "the coolest Captain around". Judging by the picture, Ellis appears to be African-American. 
While returning from a rescue mission, the unit's patient turned into a Runner, attacking their pilot and causing him to crash-land in the Colorado Mountain Plaza. Officer Larry Caulfield managed to kill the Infected but was bitten.  Following protocol, Captain Regan ordered Ellis to subdue Larry and then executed him. This left Captain Regan and Private Ellis as the only survivors.
Even with just the two left, they quickly ran out of supplies and had to search for food. They faced some Stalkers, this time Ellis himself being bitten. Panicking, Ellis attempted to take his own life, but Regan stopped him, knocked him out, and proceeded to amputate his arm.
Although Regan doubted that it had worked, Ellis was convinced his life was saved, only suffering from a fever as he recovered. As time passed, however, he became hungry and more unsettled, quickly turning violent. When Regen tried to help him eat, he lashed out, harming Regan who then fled in fear. Later the same day, the Captain returned, remembering they were family and so didn't wish to abandon him.
As time progressed, Ellis' conditioned worsened. He quickly became delusional and convinced himself that Regan wanted to kill him. While talking, Ellis saw Regan reach for her gun. Thinking she was going to kill him, he attacked her and proceeded to beat her to death. However, to his horror, he learned she had went for her weapon due to seeing Infected, who proceeded to attack Ellis, reopening his wound. He managed to escape, taking shelter in an air duct.
While there, he tried to sew his wound back up, but was unable to tie the knot due to only having one arm. He gave in and proceeded to record himself saying how much he regretted killing the Captain and wished she hadn't reached for her gun. He finished saying he was cold and wanted to rest. He died shortly after. 
Events of Left Behind Edit
When searching for medical supplies, Ellie finds the Blackhawk Ellis and his crew crashed in, which still contained a medical kit which Ellie took to heal Joel with. Ellie is also able to find Ellis' decaying corpse and the recording he made.