From Halopedia, the Halo wiki
This unidentified operator was a Spartan-II supersoldier in the United Nations Space Command's Naval Special Weapons division. This Spartan was deployed to Harvest during the Harvest campaign, where during an engagement at a Covenant Methane refinery they were killed by Sangheili Ustaf 'Nbekee with a Needler.
Conscription and augmentations
Born circa 2511, this child was abducted and conscripted into the SPARTAN-II program by Dr. Catherine Halsey in 2517 when around the time they were six years old. While on Reach, they were trained by the AI Déjà and Chief Petty Officer Mendez along with the rest of the Spartan children.
This Spartan was deployed to Harvest during the Harvest campaign. In a ground engagement on February 4, 2531, at a Methane refinery near the relic the Covenant found, this Spartan (with no backup) engaged 25 Covenant soldiers. The Spartan killed seven Covenant soldiers but was quickly killed by the Sangheili Ustaf 'Nbekee with a Needler. As a war prize, the Unggoy under 'Nbekee's command were allowed to play games with the Spartan's helmet.
It is possible that this Spartan is Joseph-122, as only his and Keiichi's status following the Harvest campaign remain unaccounted for: Three Spartans had died by that time (two by suicide and one in combat) while the remaining 28 have been named and are known to have survived the Harvest campaign.
It is unlikely that this Spartan is Keiichi-047 because despite Keiichi's status being MIA, he was seen fighting later on in the same day in a sunny period, while the unidentified Spartan was seen fighting during a dark period when Alpha Base was most likely running.