Master sergeant

Master Sergeant is the eighth enlisted rank (E-8) of the UNSC Marine Corps and the UNSC Army, and the seventh enlisted rank (E-7) in the UNSC Air Force. It is a staff non-commissioned officer rank. In the Army and Marines, it is equal in pay grade to First Sergeant, although the two have different responsibilities. The rank of Master Sergeant in the UNSC Marine Corps and Army is equal to the rank of Senior Chief Petty Officer in the UNSC Navy or Senior Master Sergeant in the Air Force. A UNSCAF Master Sergeant is equal to an Army Sergeant First Class, Marine Corps Gunnery Sergeant, or Navy Chief Petty Officer.

Master Sergeants may also be referred to by the nickname of "Top". This usage is an informal one, however, and would not be used in an official or formal setting. Use of this nickname by Marines of subordinate rank is at the rank holder's discretion.

First Sergeant and Master Sergeant
In the United States Marine Corps, eligible Gunnery Sergeants indicate whether they wish to be considered for promotion to Master Sergeant or First Sergeant, and thus whether they enter the promotional track for Master Gunnery Sergeant or Sergeant Major. Once promoted, the promotional track is permanent; lateral movement between the two programs is not possible. Whether this carries over into the UNSC Marine Corps is unconfirmed. However, lateral movement is possible in the Army.

One of the major differences between the two ranks is that First Sergeants have leadership positions, while Master Sergeants hold technical leadership billets within their MOS (job), or staff duty.

Responsibilities
When a First Sergeant is not available, a Master Sergeant may take up the role as the senior NCO for company level units. They then become the assistants and advisers for the company commander. This is seen during the events of Halo: The Flood, when Master Sergeant Lister became First Lieutenant Melissa McKay's senior NCO.