From Halopedia, the Halo wiki
A drill instructor (DI), sometimes referred to as a drill sergeant, is a non-commissioned officer specially assigned to train recruits, introducing them to the military way of life. A drill instructor is to encourage, teach, and train recruits to set themselves to the highest possible standards of personal conduct and professional skill. One of the several duties of a drill instructor is to stop exhausted and stressed recruits from taking their frustrations and anger out on each other and instead allow the recruits to take their anger out on themselves. Eventually, if the drill instructor does a proper job, the recruits will come to admire the drill instructor. Drill instructors lead physical training exercises or even participate in live-fire practice exercises (using TTR or stun rounds) against their own recruits.
Drill instructors are used in the several branches of the UNSC Armed Forces, including the UNSC Marine Corps and the Navy. Drill instructors are also found in Officer Candidate Schools, including Luna OCS Academy. The Colonial Militia's drill instructors are non-commissioned officers that serve in other branches of the UNSC and train a planetary paramilitary force. In the SPARTAN-II program, there was a naval drill instructor for each individual Spartan trainee. The Spartan-IIs were introduced to their drill instructors the day after they were kidnapped from their homes and brought to Reach. At the beginning of the SPARTAN-III program, there were only seventy-two hand-picked drill instructors for over three hundred trainees. Some of the DIs for SPARTAN-III Beta Company were medics and washouts from the preceding Alpha Company. The drill instructors of the SPARTAN-III program occasionally donned Mark II Semi-Powered Infiltration armor during live-fire training exercises. Chief Petty Officer Franklin Mendez led the drill instructors that trained both the Spartan-IIs and the Spartan-IIIs.
Drill instructors serving in the UNSC Marine Corps and UNSC Navy are occasionally referred to as "drill sergeants". However, in the United States Armed Forces—which the UNSC Armed Forces derive significant inspiration from—the term "drill sergeants" is used to only refer to drill instructors serving in the United States Army.
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