Sergeant First Class is the seventh enlisted rank (E-7) of the UNSC Army, and is a non-commissioned officer rank. It is above Staff Sergeant and below Master Sergeant and First Sergeant. It is equal to the rank of Chief Petty Officer in the UNSC Navy, Master Sergeant in the UNSC Air Force, and Gunnery Sergeant in the UNSC Marine Corps.
A Sergeant First Class typically serves as the platoon sergeant for his platoon. The platoon sergeant is second in command of the platoon, and is the highest ranking NCO in the platoon. In addition to assisting the platoon leader, he/she assists the First Sergeant on matters involving the enlisted personnel of the platoon.