Corporal
From Halopedia, the Halo wiki
Corporal is the fourth enlisted rank in the UNSC Marine Corps and the UNSC Army, and is the lowest rank of the Non-Commissioned Officer (NCO). It is equal in rank to Petty Officer Third Class in the UNSC Navy and Senior Airman in the Air Force.
Promotion to Corporal traditionally confers a large jump in authority and responsibility compared to promotion from Private through Lance Corporal.
While both the UNSC Army and the UNSC Marine Corps share the rank of Corporal, the UNSC Marine Corps promotes Lance Corporals directly to Corporal. Most of the time in the Army, Privates First Class (which are E-3) are promoted to Specialist, and then directly to Sergeant. In some cases, however, they may get promoted to Corporal instead of Specialist depending on combat tours or other military jobs.
Sources
