Jun-A266 is a Spartan-III super-solder born on the human colony world of New Harmony in 2524. When he was young, Jun was orphaned and eventually conscripted into Alpha Company of the Spartan-III Program in 2531. Following his physical augmentations in 2536, Jun was removed from the ranks of Alpha Company to become a Headhunter, covert teams sent in behind enemy lines to perform dangerous and sometimes suicidal missions.
By becoming a Headhunter, Jun was saved from participating in Operation: PROMETHEUS, a mission assigned the Alpha Company that saw the loss of every Spartan-III on the mission. Jun continued his military career as a Headhunter, eventually transitioning to Noble Team some time late in the war. Most notably, Jun and Noble Team fought in the Fall of Reach, which saw the loss of every member of Noble Team except for Jun.
Following the war, Jun was asked to train the next generation of Spartans, the Spartan-IVs, who were the first Spartans to be recruited as adults rather than conscripted as child-soldiers. For the next six years Jun trained several classes of Spartan-IVs, notably Sarah Palmer, Edward Buck, Jameson Locke, and several hundred other Spartan-IVs. In 2554, during the training of the second class of Spartan-IVs, Jun nearly died fighting off an insurrectionist terrorist on-board the Spartan-IV training station. He survived, however, and continued to train Spartans through the Rule of the Created.