Jeffries was a crew member on board the UNSC destroyer UNSC Armageddon's Edge. He had a wife and two children. Although he was laid-back in his behavior, he was incredibly skilled in Pelican piloting. He piloted the Pelican Gamma 54.
After the Armageddon's Edge arrived at Chi Rho in the Ectanus 45 system, Jeffries flew Lieutenant Jacob Keyes to the Bacigalupi Memorial Nature Preserve so that Private Tom Gerencer could drive Keyes to Camp Patmos. 
Despite disapproving of Jeffries' showing off his flying skills, Keyes was impressed by Jeffries' piloting ability. In fact, during his meeting with Commander Dmitri Zheng and Vice Admiral Jean Mawikizi, Keyes requested that Jeffries be transferred to the frigate UNSC Midsummer Night, which they would use in a mission to suppress Jason Kincaide's rioting on Charybdis IX. After Keyes informed him of this, Jeffries warned him of rumors he heard about Zheng's past. Keyes then lectured Jeffries on his habit of not following scheduled flight plans. 
On board the Midsummer Night, Jeffries became fast friends with fellow Pelican pilot Finlay. Two days after joining the Midsummer Night, Jeffries flew a platoon of ODSTs to the civilian cargo hauler Finnegan's Wake. When insurrectionist explosives destroyed Gamma 54, Jeffries was lightly wounded. ODSTs pumped biofoam into him and stabilized him. Shortly afterward, he was sent back to the Midsummer Night, where he made a full recovery. He later helped Faison, Chesnick, and the other surviving ODSTs prank Keyes: Jeffries' condition was reported to Keyes as serious, so the lieutenant (feeling guilty for transferring Jeffries to the ship) rushed to the medical bay, where he was tackled by the ODSTs, who tattooed him as a show of how impressed they were with how he handled the attack on the Finnegan's Wake. 
Shortly afterward, Jeffries piloted a new Pelican and transported Major Akio Watanabe, Jacob Keyes, and other officers to their mission to suppress Jason Kincaide's riot in the Scyllion Warehouse District on Charybdis IX. Later, Jeffries' dropship was shot down by Insurrectionists during an attempt to extract Lieutenant Keyes after the mission went wrong. Keyes was greatly saddened by Jeffries' death, especially since he was the one who had Jeffries transferred to the Midsummer Night. 
List of appearances